diff --git a/src/index.css b/src/index.css
index d6c47f4..bec41fe 100644
--- a/src/index.css
+++ b/src/index.css
@@ -5,10 +5,10 @@
 @tailwind components;
 @tailwind utilities;
 
-/* Custom scrollbar styling */
+/* Custom scrollbar styling - uses theme variables */
 * {
   scrollbar-width: thin;
-  scrollbar-color: rgba(255, 255, 255, 0.2) transparent;
+  scrollbar-color: hsl(var(--scrollbar-thumb)) hsl(var(--scrollbar-track));
 }
 
 *::-webkit-scrollbar {
@@ -17,20 +17,21 @@
 }
 
 *::-webkit-scrollbar-track {
-  background: transparent;
+  background: hsl(var(--scrollbar-track));
 }
 
 *::-webkit-scrollbar-thumb {
-  background-color: rgba(255, 255, 255, 0.2);
+  background-color: hsl(var(--scrollbar-thumb));
   border-radius: 4px;
 }
 
 *::-webkit-scrollbar-thumb:hover {
-  background-color: rgba(255, 255, 255, 0.3);
+  background-color: hsl(var(--scrollbar-thumb-hover));
 }
 
 @layer base {
   :root {
+    /* Core colors - light theme defaults (overridden by ThemeProvider) */
     --background: 0 0% 100%;
     --foreground: 222.2 84% 4.9%;
     --card: 0 0% 100%;
@@ -51,12 +52,43 @@
     --input: 214.3 31.8% 91.4%;
     --ring: 222.2 84% 4.9%;
     --radius: 0.5rem;
-    --chart-1: 12 76% 61%;
-    --chart-2: 173 58% 39%;
-    --chart-3: 197 37% 24%;
-    --chart-4: 43 74% 66%;
-    --chart-5: 27 87% 67%;
+
+    /* Status colors */
+    --success: 142 76% 36%;
+    --warning: 45 93% 47%;
+    --info: 199 89% 48%;
+
+    /* Syntax highlighting */
+    --syntax-comment: 215.4 16.3% 46.9%;
+    --syntax-punctuation: 222.2 84% 30%;
+    --syntax-property: 222.2 47.4% 11.2%;
+    --syntax-string: 142 60% 30%;
+    --syntax-keyword: 270 80% 50%;
+    --syntax-function: 222.2 47.4% 11.2%;
+    --syntax-variable: 222.2 84% 4.9%;
+    --syntax-operator: 222.2 84% 20%;
+
+    /* Diff colors */
+    --diff-inserted: 142 60% 30%;
+    --diff-inserted-bg: 142 60% 50% / 0.15;
+    --diff-deleted: 0 70% 45%;
+    --diff-deleted-bg: 0 70% 50% / 0.15;
+    --diff-meta: 199 80% 40%;
+    --diff-meta-bg: 199 80% 50% / 0.1;
+
+    /* Scrollbar */
+    --scrollbar-thumb: 222.2 84% 4.9% / 0.2;
+    --scrollbar-thumb-hover: 222.2 84% 4.9% / 0.3;
+    --scrollbar-track: 0 0% 0% / 0;
+
+    /* Gradient colors (RGB values) */
+    --gradient-1: 234 179 8;
+    --gradient-2: 249 115 22;
+    --gradient-3: 147 51 234;
+    --gradient-4: 6 182 212;
   }
+
+  /* Dark theme - applied via .dark class or ThemeProvider */
   .dark {
     --background: 222.2 84% 4.9%;
     --foreground: 210 40% 98%;
@@ -77,11 +109,40 @@
     --border: 217.2 32.6% 17.5%;
     --input: 217.2 32.6% 17.5%;
     --ring: 212.7 26.8% 83.9%;
-    --chart-1: 220 70% 50%;
-    --chart-2: 160 60% 45%;
-    --chart-3: 30 80% 55%;
-    --chart-4: 280 65% 60%;
-    --chart-5: 340 75% 55%;
+
+    /* Status colors */
+    --success: 142 76% 36%;
+    --warning: 45 93% 47%;
+    --info: 199 89% 48%;
+
+    /* Syntax highlighting */
+    --syntax-comment: 215 20.2% 70%;
+    --syntax-punctuation: 210 40% 70%;
+    --syntax-property: 210 40% 98%;
+    --syntax-string: 215 20.2% 70%;
+    --syntax-keyword: 210 40% 98%;
+    --syntax-function: 210 40% 98%;
+    --syntax-variable: 210 40% 98%;
+    --syntax-operator: 210 40% 98%;
+
+    /* Diff colors */
+    --diff-inserted: 134 60% 76%;
+    --diff-inserted-bg: 145 63% 42% / 0.1;
+    --diff-deleted: 0 100% 76%;
+    --diff-deleted-bg: 0 100% 60% / 0.1;
+    --diff-meta: 190 77% 70%;
+    --diff-meta-bg: 190 77% 70% / 0.08;
+
+    /* Scrollbar */
+    --scrollbar-thumb: 0 0% 100% / 0.2;
+    --scrollbar-thumb-hover: 0 0% 100% / 0.3;
+    --scrollbar-track: 0 0% 0% / 0;
+
+    /* Gradient colors (RGB values) */
+    --gradient-1: 250 204 21;
+    --gradient-2: 251 146 60;
+    --gradient-3: 168 85 247;
+    --gradient-4: 34 211 238;
   }
 }
 
@@ -106,10 +167,10 @@
   .text-grimoire-gradient {
     background: linear-gradient(
       to bottom,
-      rgb(250 204 21),
-      /* yellow-400 */ rgb(251 146 60),
-      /* orange-400 */ rgb(168 85 247),
-      /* purple-500 */ rgb(34 211 238) /* cyan-400 */
+      rgb(var(--gradient-1)),
+      rgb(var(--gradient-2)),
+      rgb(var(--gradient-3)),
+      rgb(var(--gradient-4))
     );
     background-clip: text;
     -webkit-background-clip: text;
@@ -117,16 +178,16 @@
   }
 }
 
-/* react-medium-image-zoom dark theme customization */
+/* react-medium-image-zoom theme customization - uses background with opacity */
 [data-rmiz-modal-overlay] {
-  background-color: rgba(12, 12, 18, 0.92) !important;
+  background-color: hsl(var(--background) / 0.92) !important;
 }
 
 [data-rmiz-modal-content] {
-  box-shadow: 0 0 40px rgba(0, 0, 0, 0.5);
+  box-shadow: 0 0 40px hsl(var(--foreground) / 0.2);
 }
 
-/* React Mosaic Dark Theme Customization */
+/* React Mosaic Theme Customization */
 .mosaic.mosaic-blueprint-theme.mosaic-blueprint-theme {
   background: hsl(var(--background));
 }
diff --git a/src/lib/themes/apply.ts b/src/lib/themes/apply.ts
new file mode 100644
index 0000000..a64ba4a
--- /dev/null
+++ b/src/lib/themes/apply.ts
@@ -0,0 +1,150 @@
+import type { Theme } from "./types";
+
+/**
+ * Apply a theme by setting CSS custom properties on the document root.
+ * This updates all theme variables at runtime without requiring a page reload.
+ */
+export function applyTheme(theme: Theme): void {
+  const root = document.documentElement;
+
+  // Apply core colors
+  root.style.setProperty("--background", theme.colors.background);
+  root.style.setProperty("--foreground", theme.colors.foreground);
+
+  root.style.setProperty("--card", theme.colors.card);
+  root.style.setProperty("--card-foreground", theme.colors.cardForeground);
+
+  root.style.setProperty("--popover", theme.colors.popover);
+  root.style.setProperty(
+    "--popover-foreground",
+    theme.colors.popoverForeground,
+  );
+
+  root.style.setProperty("--primary", theme.colors.primary);
+  root.style.setProperty(
+    "--primary-foreground",
+    theme.colors.primaryForeground,
+  );
+
+  root.style.setProperty("--secondary", theme.colors.secondary);
+  root.style.setProperty(
+    "--secondary-foreground",
+    theme.colors.secondaryForeground,
+  );
+
+  root.style.setProperty("--accent", theme.colors.accent);
+  root.style.setProperty("--accent-foreground", theme.colors.accentForeground);
+
+  root.style.setProperty("--muted", theme.colors.muted);
+  root.style.setProperty("--muted-foreground", theme.colors.mutedForeground);
+
+  root.style.setProperty("--destructive", theme.colors.destructive);
+  root.style.setProperty(
+    "--destructive-foreground",
+    theme.colors.destructiveForeground,
+  );
+
+  root.style.setProperty("--border", theme.colors.border);
+  root.style.setProperty("--input", theme.colors.input);
+  root.style.setProperty("--ring", theme.colors.ring);
+
+  // Status colors
+  root.style.setProperty("--success", theme.colors.success);
+  root.style.setProperty("--warning", theme.colors.warning);
+  root.style.setProperty("--info", theme.colors.info);
+
+  // Syntax highlighting
+  root.style.setProperty("--syntax-comment", theme.syntax.comment);
+  root.style.setProperty("--syntax-punctuation", theme.syntax.punctuation);
+  root.style.setProperty("--syntax-property", theme.syntax.property);
+  root.style.setProperty("--syntax-string", theme.syntax.string);
+  root.style.setProperty("--syntax-keyword", theme.syntax.keyword);
+  root.style.setProperty("--syntax-function", theme.syntax.function);
+  root.style.setProperty("--syntax-variable", theme.syntax.variable);
+  root.style.setProperty("--syntax-operator", theme.syntax.operator);
+
+  // Diff colors
+  root.style.setProperty("--diff-inserted", theme.syntax.diffInserted);
+  root.style.setProperty("--diff-inserted-bg", theme.syntax.diffInsertedBg);
+  root.style.setProperty("--diff-deleted", theme.syntax.diffDeleted);
+  root.style.setProperty("--diff-deleted-bg", theme.syntax.diffDeletedBg);
+  root.style.setProperty("--diff-meta", theme.syntax.diffMeta);
+  root.style.setProperty("--diff-meta-bg", theme.syntax.diffMetaBg);
+
+  // Scrollbar
+  root.style.setProperty("--scrollbar-thumb", theme.scrollbar.thumb);
+  root.style.setProperty("--scrollbar-thumb-hover", theme.scrollbar.thumbHover);
+  root.style.setProperty("--scrollbar-track", theme.scrollbar.track);
+
+  // Gradient
+  root.style.setProperty("--gradient-1", theme.gradient.color1);
+  root.style.setProperty("--gradient-2", theme.gradient.color2);
+  root.style.setProperty("--gradient-3", theme.gradient.color3);
+  root.style.setProperty("--gradient-4", theme.gradient.color4);
+
+  // Layout
+  root.style.setProperty("--radius", theme.radius);
+
+  // Remove the dark class management - we now handle this via CSS variables directly
+  // The dark class is no longer needed as themes apply their own color values
+}
+
+/**
+ * Get all CSS variable names used by the theme system.
+ * Useful for debugging or documentation.
+ */
+export function getThemeVariables(): string[] {
+  return [
+    // Core colors
+    "--background",
+    "--foreground",
+    "--card",
+    "--card-foreground",
+    "--popover",
+    "--popover-foreground",
+    "--primary",
+    "--primary-foreground",
+    "--secondary",
+    "--secondary-foreground",
+    "--accent",
+    "--accent-foreground",
+    "--muted",
+    "--muted-foreground",
+    "--destructive",
+    "--destructive-foreground",
+    "--border",
+    "--input",
+    "--ring",
+    // Status
+    "--success",
+    "--warning",
+    "--info",
+    // Syntax
+    "--syntax-comment",
+    "--syntax-punctuation",
+    "--syntax-property",
+    "--syntax-string",
+    "--syntax-keyword",
+    "--syntax-function",
+    "--syntax-variable",
+    "--syntax-operator",
+    // Diff
+    "--diff-inserted",
+    "--diff-inserted-bg",
+    "--diff-deleted",
+    "--diff-deleted-bg",
+    "--diff-meta",
+    "--diff-meta-bg",
+    // Scrollbar
+    "--scrollbar-thumb",
+    "--scrollbar-thumb-hover",
+    "--scrollbar-track",
+    // Gradient
+    "--gradient-1",
+    "--gradient-2",
+    "--gradient-3",
+    "--gradient-4",
+    // Layout
+    "--radius",
+  ];
+}
diff --git a/src/lib/themes/builtin/dark.ts b/src/lib/themes/builtin/dark.ts
new file mode 100644
index 0000000..290ff47
--- /dev/null
+++ b/src/lib/themes/builtin/dark.ts
@@ -0,0 +1,80 @@
+import type { Theme } from "../types";
+
+/**
+ * Dark theme - the original Grimoire theme
+ * Deep blue-black background with bright purple accent
+ */
+export const darkTheme: Theme = {
+  id: "dark",
+  name: "Dark",
+  description: "The original Grimoire dark theme",
+
+  colors: {
+    background: "222.2 84% 4.9%",
+    foreground: "210 40% 98%",
+
+    card: "222.2 84% 4.9%",
+    cardForeground: "210 40% 98%",
+
+    popover: "222.2 84% 4.9%",
+    popoverForeground: "210 40% 98%",
+
+    primary: "210 40% 98%",
+    primaryForeground: "222.2 47.4% 11.2%",
+
+    secondary: "217.2 32.6% 17.5%",
+    secondaryForeground: "210 40% 98%",
+
+    accent: "270 100% 70%",
+    accentForeground: "222.2 84% 4.9%",
+
+    muted: "217.2 32.6% 17.5%",
+    mutedForeground: "215 20.2% 70%",
+
+    destructive: "0 62.8% 30.6%",
+    destructiveForeground: "210 40% 98%",
+
+    border: "217.2 32.6% 17.5%",
+    input: "217.2 32.6% 17.5%",
+    ring: "212.7 26.8% 83.9%",
+
+    // Status colors
+    success: "142 76% 36%",
+    warning: "45 93% 47%",
+    info: "199 89% 48%",
+  },
+
+  syntax: {
+    comment: "215 20.2% 70%",
+    punctuation: "210 40% 70%",
+    property: "210 40% 98%",
+    string: "215 20.2% 70%",
+    keyword: "210 40% 98%",
+    function: "210 40% 98%",
+    variable: "210 40% 98%",
+    operator: "210 40% 98%",
+
+    // Diff colors (converted from hardcoded RGB)
+    diffInserted: "134 60% 76%",
+    diffInsertedBg: "145 63% 42% / 0.1",
+    diffDeleted: "0 100% 76%",
+    diffDeletedBg: "0 100% 60% / 0.1",
+    diffMeta: "190 77% 70%",
+    diffMetaBg: "190 77% 70% / 0.08",
+  },
+
+  scrollbar: {
+    thumb: "0 0% 100% / 0.2",
+    thumbHover: "0 0% 100% / 0.3",
+    track: "0 0% 0% / 0",
+  },
+
+  gradient: {
+    color1: "250 204 21", // yellow-400
+    color2: "251 146 60", // orange-400
+    color3: "168 85 247", // purple-500
+    color4: "34 211 238", // cyan-400
+  },
+
+  radius: "0.5rem",
+};
diff --git a/src/lib/themes/builtin/index.ts b/src/lib/themes/builtin/index.ts
new file mode 100644
index 0000000..cd06c79
--- /dev/null
+++ b/src/lib/themes/builtin/index.ts
@@ -0,0 +1,23 @@
+export { darkTheme } from "./dark";
+export { lightTheme } from "./light";
+export { plan9Theme } from "./plan9";
+
+import { darkTheme } from "./dark";
+import { lightTheme } from "./light";
+import { plan9Theme } from "./plan9";
+import type { Theme, BuiltinThemeId } from "../types";
+
+/** Map of all built-in themes by ID */
+export const builtinThemes: Record<BuiltinThemeId, Theme> = {
+  dark: darkTheme,
+  light: lightTheme,
+  plan9: plan9Theme,
+};
+
+/** Array of all built-in themes for iteration */
+export const builtinThemeList: Theme[] = [darkTheme, lightTheme, plan9Theme];
+
+/** Get a built-in theme by ID */
+export function getBuiltinTheme(id: BuiltinThemeId): Theme {
+  return builtinThemes[id];
+}
diff --git a/src/lib/themes/builtin/light.ts b/src/lib/themes/builtin/light.ts
new file mode 100644
index 0000000..942aa8f
--- /dev/null
+++ b/src/lib/themes/builtin/light.ts
@@ -0,0 +1,80 @@
+import type { Theme } from "../types";
+
+/**
+ * Light theme - clean white background
+ * Based on the original shadcn/ui light mode values
+ */
+export const lightTheme: Theme = {
+  id: "light",
+  name: "Light",
+  description: "Clean light theme for daytime use",
+
+  colors: {
+    background: "0 0% 100%",
+    foreground: "222.2 84% 4.9%",
+
+    card: "0 0% 100%",
+    cardForeground: "222.2 84% 4.9%",
+
+    popover: "0 0% 100%",
+    popoverForeground: "222.2 84% 4.9%",
+
+    primary: "222.2 47.4% 11.2%",
+    primaryForeground: "210 40% 98%",
+
+    secondary: "210 40% 96.1%",
+    secondaryForeground: "222.2 47.4% 11.2%",
+
+    accent: "270 80% 60%",
+    accentForeground: "0 0% 100%",
+
+    muted: "210 40% 96.1%",
+    mutedForeground: "215.4 16.3% 46.9%",
+
+    destructive: "0 84.2% 60.2%",
+    destructiveForeground: "210 40% 98%",
+
+    border: "214.3 31.8% 91.4%",
+    input: "214.3 31.8% 91.4%",
+    ring: "222.2 84% 4.9%",
+
+    // Status colors
+    success: "142 76% 36%",
+    warning: "45 93% 47%",
+    info: "199 89% 48%",
+  },
+
+  syntax: {
+    comment: "215.4 16.3% 46.9%",
+    punctuation: "222.2 84% 30%",
+    property: "222.2 47.4% 11.2%",
+    string: "142 60% 30%",
+    keyword: "270 80% 50%",
+    function: "222.2 47.4% 11.2%",
+    variable: "222.2 84% 4.9%",
+    operator: "222.2 84% 20%",
+
+    // Diff colors
+    diffInserted: "142 60% 30%",
+    diffInsertedBg: "142 60% 50% / 0.15",
+    diffDeleted: "0 70% 45%",
+    diffDeletedBg: "0 70% 50% / 0.15",
+    diffMeta: "199 80% 40%",
+    diffMetaBg: "199 80% 50% / 0.1",
+  },
+
+  scrollbar: {
+    thumb: "222.2 84% 4.9% / 0.2",
+    thumbHover: "222.2 84% 4.9% / 0.3",
+    track: "0 0% 0% / 0",
+  },
+
+  gradient: {
+    color1: "234 179 8", // yellow-500 (darker for light bg)
+    color2: "249 115 22", // orange-500
+    color3: "147 51 234", // purple-600
+    color4: "6 182 212", // cyan-500
+  },
+
+  radius: "0.5rem",
+};
diff --git a/src/lib/themes/builtin/plan9.ts b/src/lib/themes/builtin/plan9.ts
new file mode 100644
index 0000000..1302e57
--- /dev/null
+++ b/src/lib/themes/builtin/plan9.ts
@@ -0,0 +1,98 @@
+import type { Theme } from "../types";
+
+/**
+ * Plan 9 theme - inspired by the Plan 9 from Bell Labs operating system
+ *
+ * Characteristics:
+ * - Pale yellow/cream backgrounds (#ffffe0)
+ * - Light blue-green window chrome (#eaffea)
+ * - Black text for high contrast
+ * - Bright yellow selections
+ * - Dark blue accents
+ * - No rounded corners (squared aesthetic)
+ */
+export const plan9Theme: Theme = {
+  id: "plan9",
+  name: "Plan 9",
+  description: "Inspired by Plan 9 from Bell Labs",
+
+  colors: {
+    // Characteristic pale yellow background
+    background: "60 100% 94%", // #ffffe0
+    foreground: "0 0% 0%", // Pure black
+
+    // Window chrome - pale green (acme editor style)
+    card: "120 100% 95%", // #eaffea
+    cardForeground: "0 0% 0%",
+
+    popover: "60 100% 97%",
+    popoverForeground: "0 0% 0%",
+
+    // Dark blue for interactive elements
+    primary: "220 100% 25%",
+    primaryForeground: "60 100% 94%",
+
+    // Muted green secondary
+    secondary: "120 30% 88%",
+    secondaryForeground: "0 0% 0%",
+
+    // Bright yellow accent (Plan9 signature selection color)
+    accent: "60 100% 50%",
+    accentForeground: "0 0% 0%",
+
+    // Muted yellow for subdued elements
+    muted: "60 30% 88%",
+    mutedForeground: "0 0% 35%",
+
+    // Red for destructive
+    destructive: "0 70% 45%",
+    destructiveForeground: "0 0% 100%",
+
+    // Dark blue borders
+    border: "220 40% 50%",
+    input: "60 30% 92%",
+    ring: "220 100% 25%",
+
+    // Status colors
+    success: "120 60% 30%",
+    warning: "45 90% 45%",
+    info: "200 80% 40%",
+  },
+
+  syntax: {
+    // Acme-inspired syntax colors
+    comment: "0 0% 45%", // Gray
+    punctuation: "0 0% 25%", // Dark gray
+    property: "220 100% 25%", // Dark blue
+    string: "120 60% 28%", // Forest green
+    keyword: "280 60% 35%", // Purple
+    function: "220 100% 25%", // Dark blue
+    variable: "0 0% 0%", // Black
+    operator: "0 0% 15%", // Near black
+
+    // Diff colors - subtle on yellow background
+    diffInserted: "120 60% 25%",
+    diffInsertedBg: "120 50% 85%",
+    diffDeleted: "0 65% 40%",
+    diffDeletedBg: "0 50% 90%",
+    diffMeta: "200 70% 35%",
+    diffMetaBg: "200 50% 88%",
+  },
+
+  scrollbar: {
+    thumb: "220 30% 55%",
+    thumbHover: "220 40% 45%",
+    track: "60 30% 90%",
+  },
+
+  gradient: {
+    // Muted gradient for Plan9 aesthetic
+    color1: "180 140 20", // Olive/mustard
+    color2: "200 120 50", // Burnt orange
+    color3: "100 60 180", // Muted purple
+    color4: "40 160 180", // Teal
+  },
+
+  // Plan9 has no rounded corners - everything is squared
+  radius: "0",
+};
diff --git a/src/lib/themes/context.tsx b/src/lib/themes/context.tsx
new file mode 100644
index 0000000..745ebae
--- /dev/null
+++ b/src/lib/themes/context.tsx
@@ -0,0 +1,193 @@
+import * as React from "react";
+import type { Theme, BuiltinThemeId } from "./types";
+import { isBuiltinTheme } from "./types";
+import { builtinThemes, builtinThemeList } from "./builtin";
+import { applyTheme } from "./apply";
+
+const STORAGE_KEY = "grimoire-theme";
+const DEFAULT_THEME_ID: BuiltinThemeId = "dark";
+
+interface ThemeContextValue {
+  /** Current active theme */
+  theme: Theme;
+  /** Current theme ID */
+  themeId: string;
+  /** Set theme by ID */
+  setTheme: (id: string) => void;
+  /** List of all available themes (builtin + custom) */
+  availableThemes: Theme[];
+  /** Custom themes added by user */
+  customThemes: Theme[];
+  /** Add a custom theme */
+  addCustomTheme: (theme: Theme) => void;
+  /** Remove a custom theme by ID */
+  removeCustomTheme: (id: string) => void;
+}
+
+const ThemeContext = React.createContext<ThemeContextValue | null>(null);
+
+/**
+ * Hook to access theme context
+ * Must be used within a ThemeProvider
+ */
+export function useTheme(): ThemeContextValue {
+  const context = React.useContext(ThemeContext);
+  if (!context) {
+    throw new Error("useTheme must be used within a ThemeProvider");
+  }
+  return context;
+}
+
+/**
+ * Get the saved theme ID from localStorage
+ */
+function getSavedThemeId(): string {
+  try {
+    const saved = localStorage.getItem(STORAGE_KEY);
+    if (saved) {
+      const data = JSON.parse(saved);
+      return data.themeId || DEFAULT_THEME_ID;
+    }
+  } catch {
+    // Ignore parse errors
+  }
+  return DEFAULT_THEME_ID;
+}
+
+/**
+ * Get saved custom themes from localStorage
+ */
+function getSavedCustomThemes(): Theme[] {
+  try {
+    const saved = localStorage.getItem(STORAGE_KEY);
+    if (saved) {
+      const data = JSON.parse(saved);
+      return data.customThemes || [];
+    }
+  } catch {
+    // Ignore parse errors
+  }
+  return [];
+}
+
+/**
+ * Save theme data to localStorage
+ */
+function saveThemeData(themeId: string, customThemes: Theme[]): void {
+  try {
+    localStorage.setItem(
+      STORAGE_KEY,
+      JSON.stringify({ themeId, customThemes }),
+    );
+  } catch {
+    // Ignore storage errors (quota exceeded, etc.)
+  }
+}
+
+/**
+ * Find a theme by ID from builtin and custom themes
+ */
+function findTheme(id: string, customThemes: Theme[]): Theme | undefined {
+  if (isBuiltinTheme(id)) {
+    return builtinThemes[id];
+  }
+  return customThemes.find((t) => t.id === id);
+}
+
+interface ThemeProviderProps {
+  children: React.ReactNode;
+  /** Default theme ID (overrides localStorage on first render) */
+  defaultTheme?: string;
+}
+
+/**
+ * Theme provider component
+ * Manages theme state, persistence, and CSS variable application
+ */
+export function ThemeProvider({
+  children,
+  defaultTheme,
+}: ThemeProviderProps): React.ReactElement {
+  // Initialize from localStorage or default
+  const [themeId, setThemeIdState] = React.useState<string>(() => {
+    return defaultTheme || getSavedThemeId();
+  });
+
+  const [customThemes, setCustomThemes] = React.useState<Theme[]>(() => {
+    return getSavedCustomThemes();
+  });
+
+  // Resolve current theme
+  const theme = React.useMemo(() => {
+    return findTheme(themeId, customThemes) || builtinThemes[DEFAULT_THEME_ID];
+  }, [themeId, customThemes]);
+
+  // Apply theme on mount and when theme changes
+  React.useEffect(() => {
+    applyTheme(theme);
+  }, [theme]);
+
+  // Save to localStorage when theme changes
+  React.useEffect(() => {
+    saveThemeData(themeId, customThemes);
+  }, [themeId, customThemes]);
+
+  const setTheme = React.useCallback((id: string) => {
+    setThemeIdState(id);
+  }, []);
+
+  const addCustomTheme = React.useCallback((newTheme: Theme) => {
+    setCustomThemes((prev) => {
+      // Replace if exists, otherwise add
+      const existing = prev.findIndex((t) => t.id === newTheme.id);
+      if (existing >= 0) {
+        const updated = [...prev];
+        updated[existing] = newTheme;
+        return updated;
+      }
+      return [...prev, newTheme];
+    });
+  }, []);
+
+  const removeCustomTheme = React.useCallback(
+    (id: string) => {
+      setCustomThemes((prev) => prev.filter((t) => t.id !== id));
+      // If removing current theme, switch to default
+      if (themeId === id) {
+        setThemeIdState(DEFAULT_THEME_ID);
+      }
+    },
+    [themeId],
+  );
+
+  const availableThemes = React.useMemo(() => {
+    return [...builtinThemeList, ...customThemes];
+  }, [customThemes]);
+
+  const contextValue = React.useMemo<ThemeContextValue>(
+    () => ({
+      theme,
+      themeId,
+      setTheme,
+      availableThemes,
+      customThemes,
+      addCustomTheme,
+      removeCustomTheme,
+    }),
+    [
+      theme,
+      themeId,
+      setTheme,
+      availableThemes,
+      customThemes,
+      addCustomTheme,
+      removeCustomTheme,
+    ],
+  );
+
+  return (
+    <ThemeContext.Provider value={contextValue}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
diff --git a/src/lib/themes/index.ts b/src/lib/themes/index.ts
new file mode 100644
index 0000000..1de0865
--- /dev/null
+++ b/src/lib/themes/index.ts
@@ -0,0 +1,29 @@
+// Types
+export type {
+  Theme,
+  ThemeColors,
+  ThemeSyntax,
+  ThemeScrollbar,
+  ThemeGradient,
+  ThemeMeta,
+  HSLValue,
+  RGBValue,
+  BuiltinThemeId,
+} from "./types";
+export { isBuiltinTheme } from "./types";
+
+// Built-in themes
+export {
+  darkTheme,
+  lightTheme,
+  plan9Theme,
+  builtinThemes,
+  builtinThemeList,
+  getBuiltinTheme,
+} from "./builtin";
+
+// Theme application
+export { applyTheme, getThemeVariables } from "./apply";
+
+// Context and hooks
+export { ThemeProvider, useTheme } from "./context";
diff --git a/src/lib/themes/types.ts b/src/lib/themes/types.ts
new file mode 100644
index 0000000..6fc1746
--- /dev/null
+++ b/src/lib/themes/types.ts
@@ -0,0 +1,149 @@
+/**
+ * Theme System Types
+ *
+ * All color values are HSL without the hsl() wrapper.
+ * Format: "hue saturation% lightness%" (e.g., "220 70% 50%")
+ *
+ * For Nostr publishing:
+ * - kind: 30078 (NIP-78 arbitrary app data)
+ * - d tag: "grimoire-theme"
+ * - name tag: theme display name
+ */
+
+/** HSL color value without wrapper (e.g., "220 70% 50%") */
+export type HSLValue = string;
+
+/** RGB color for gradients (e.g., "250 204 21") */
+export type RGBValue = string;
+
+/** Core semantic colors for UI components */
+export interface ThemeColors {
+  // Core backgrounds and text
+  background: HSLValue;
+  foreground: HSLValue;
+
+  // Card/panel surfaces
+  card: HSLValue;
+  cardForeground: HSLValue;
+
+  // Popover/dropdown surfaces
+  popover: HSLValue;
+  popoverForeground: HSLValue;
+
+  // Primary interactive elements
+  primary: HSLValue;
+  primaryForeground: HSLValue;
+
+  // Secondary interactive elements
+  secondary: HSLValue;
+  secondaryForeground: HSLValue;
+
+  // Accent/highlight color
+  accent: HSLValue;
+  accentForeground: HSLValue;
+
+  // Subdued/muted elements
+  muted: HSLValue;
+  mutedForeground: HSLValue;
+
+  // Destructive/error states
+  destructive: HSLValue;
+  destructiveForeground: HSLValue;
+
+  // Form elements
+  border: HSLValue;
+  input: HSLValue;
+  ring: HSLValue;
+
+  // Status indicators (replacing hardcoded Tailwind colors)
+  success: HSLValue;
+  warning: HSLValue;
+  info: HSLValue;
+}
+
+/** Syntax highlighting colors for code blocks */
+export interface ThemeSyntax {
+  // General tokens
+  comment: HSLValue;
+  punctuation: HSLValue;
+  property: HSLValue;
+  string: HSLValue;
+  keyword: HSLValue;
+  function: HSLValue;
+  variable: HSLValue;
+  operator: HSLValue;
+
+  // Diff-specific tokens
+  diffInserted: HSLValue;
+  diffInsertedBg: HSLValue;
+  diffDeleted: HSLValue;
+  diffDeletedBg: HSLValue;
+  diffMeta: HSLValue;
+  diffMetaBg: HSLValue;
+}
+
+/** Scrollbar styling */
+export interface ThemeScrollbar {
+  thumb: HSLValue;
+  thumbHover: HSLValue;
+  track: HSLValue;
+}
+
+/** Gradient colors (RGB values for CSS rgb() function) */
+export interface ThemeGradient {
+  // Grimoire brand gradient (4 color stops)
+  color1: RGBValue; // Top - yellow
+  color2: RGBValue; // Upper-middle - orange
+  color3: RGBValue; // Lower-middle - purple
+  color4: RGBValue; // Bottom - cyan
+}
+
+/** Complete theme definition */
+export interface Theme {
+  /** Unique identifier (e.g., "plan9", "dark", "light") */
+  id: string;
+
+  /** Display name shown in UI */
+  name: string;
+
+  /** Theme author (npub or display name) */
+  author?: string;
+
+  /** Semantic version */
+  version?: string;
+
+  /** Theme description */
+  description?: string;
+
+  /** Core UI colors */
+  colors: ThemeColors;
+
+  /** Syntax highlighting colors */
+  syntax: ThemeSyntax;
+
+  /** Scrollbar colors */
+  scrollbar: ThemeScrollbar;
+
+  /** Gradient colors */
+  gradient: ThemeGradient;
+
+  /** Border radius base value (e.g., "0.5rem", "0") */
+  radius: string;
+}
+
+/** Theme metadata for listings (without full color data) */
+export interface ThemeMeta {
+  id: string;
+  name: string;
+  author?: string;
+  version?: string;
+  description?: string;
+}
+
+/** Built-in theme IDs */
+export type BuiltinThemeId = "dark" | "light" | "plan9";
+
+/** Check if a theme ID is a built-in theme */
+export function isBuiltinTheme(id: string): id is BuiltinThemeId {
+  return id === "dark" || id === "light" || id === "plan9";
+}
diff --git a/src/main.tsx b/src/main.tsx
index 0ea432d..dd08235 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -8,31 +8,30 @@ import { Toaster } from "sonner";
 import { TooltipProvider } from "./components/ui/tooltip";
 import { ErrorBoundary } from "./components/ErrorBoundary";
 import { initializeErrorHandling } from "./lib/error-handler";
-
-// Add dark class to html element for default dark theme
-document.documentElement.classList.add("dark");
+import { ThemeProvider } from "./lib/themes";
 
 // Initialize global error handling
 initializeErrorHandling();
 
 createRoot(document.getElementById("root")!).render(
   <ErrorBoundary level="app">
-    <EventStoreProvider eventStore={eventStore}>
-      <TooltipProvider>
-        <Toaster
-          position="top-center"
-          theme="dark"
-          toastOptions={{
-            style: {
-              background: "hsl(var(--background))",
-              color: "hsl(var(--foreground))",
-              border: "1px solid hsl(var(--border))",
-              borderRadius: 0,
-            },
-          }}
-        />
-        <Root />
-      </TooltipProvider>
-    </EventStoreProvider>
+    <ThemeProvider defaultTheme="dark">
+      <EventStoreProvider eventStore={eventStore}>
+        <TooltipProvider>
+          <Toaster
+            position="top-center"
+            toastOptions={{
+              style: {
+                background: "hsl(var(--background))",
+                color: "hsl(var(--foreground))",
+                border: "1px solid hsl(var(--border))",
+                borderRadius: "var(--radius)",
+              },
+            }}
+          />
+          <Root />
+        </TooltipProvider>
+      </EventStoreProvider>
+    </ThemeProvider>
   </ErrorBoundary>,
 );
diff --git a/src/styles/prism-theme.css b/src/styles/prism-theme.css
index 168517f..a3f9dfe 100644
--- a/src/styles/prism-theme.css
+++ b/src/styles/prism-theme.css
@@ -1,4 +1,4 @@
-/* Grimoire Prism Theme - Matches dark theme using CSS variables */
+/* Grimoire Prism Theme - Uses CSS theme variables */
 
 code[class*="language-"],
 pre[class*="language-"] {
@@ -19,8 +19,8 @@ pre[class*="language-"] {
 
 /* Deleted lines (red) - subtle background, no strikethrough */
 .token.deleted {
-  color: #ff8787;
-  background: rgba(255, 59, 48, 0.1);
+  color: hsl(var(--diff-deleted));
+  background: hsl(var(--diff-deleted-bg));
   display: block;
   margin: 0 -1rem;
   padding: 0 1rem;
@@ -28,8 +28,8 @@ pre[class*="language-"] {
 
 /* Added lines (green) - subtle background */
 .token.inserted {
-  color: #69db7c;
-  background: rgba(52, 199, 89, 0.1);
+  color: hsl(var(--diff-inserted));
+  background: hsl(var(--diff-inserted-bg));
   display: block;
   margin: 0 -1rem;
   padding: 0 1rem;
@@ -38,8 +38,8 @@ pre[class*="language-"] {
 /* Hunk headers (@@ -1,5 +1,7 @@) - cyan/blue */
 .token.diff.coord,
 .token.coord {
-  color: #66d9ef;
-  background: rgba(102, 217, 239, 0.08);
+  color: hsl(var(--diff-meta));
+  background: hsl(var(--diff-meta-bg));
   display: block;
   margin: 0 -1rem;
   padding: 0 1rem;
@@ -66,11 +66,11 @@ pre[class*="language-"] {
 .token.prolog,
 .token.doctype,
 .token.cdata {
-  color: hsl(var(--muted-foreground));
+  color: hsl(var(--syntax-comment));
 }
 
 .token.punctuation {
-  color: hsl(var(--foreground) / 0.7);
+  color: hsl(var(--syntax-punctuation));
 }
 
 .token.property,
@@ -79,7 +79,7 @@ pre[class*="language-"] {
 .token.number,
 .token.constant,
 .token.symbol {
-  color: hsl(var(--primary));
+  color: hsl(var(--syntax-property));
 }
 
 .token.selector,
@@ -87,8 +87,7 @@ pre[class*="language-"] {
 .token.string,
 .token.char,
 .token.builtin {
-  color: hsl(var(--muted-foreground));
-  font-weight: 500;
+  color: hsl(var(--syntax-string));
 }
 
 .token.operator,
@@ -96,25 +95,25 @@ pre[class*="language-"] {
 .token.url,
 .language-css .token.string,
 .style .token.string {
-  color: hsl(var(--foreground));
+  color: hsl(var(--syntax-operator));
 }
 
 .token.atrule,
 .token.attr-value,
 .token.keyword {
-  color: hsl(var(--primary));
+  color: hsl(var(--syntax-keyword));
 }
 
 .token.function,
 .token.class-name {
-  color: hsl(var(--primary));
+  color: hsl(var(--syntax-function));
   font-weight: bold;
 }
 
 .token.regex,
 .token.important,
 .token.variable {
-  color: hsl(var(--primary));
+  color: hsl(var(--syntax-variable));
 }
 
 .token.important,
diff --git a/tailwind.config.js b/tailwind.config.js
index 23c5e58..df78150 100644
--- a/tailwind.config.js
+++ b/tailwind.config.js
@@ -65,13 +65,10 @@ export default {
         border: "hsl(var(--border))",
         input: "hsl(var(--input))",
         ring: "hsl(var(--ring))",
-        chart: {
-          1: "hsl(var(--chart-1))",
-          2: "hsl(var(--chart-2))",
-          3: "hsl(var(--chart-3))",
-          4: "hsl(var(--chart-4))",
-          5: "hsl(var(--chart-5))",
-        },
+        // Status colors
+        success: "hsl(var(--success))",
+        warning: "hsl(var(--warning))",
+        info: "hsl(var(--info))",
       },
     },
   },