feat(theme): add CSS variable tokens and theme classes

- Define semantic color tokens (--color-bg, --color-text, etc.)
- Add .theme-dark, .theme-light, .theme-system CSS classes
- Create theme.ts utility for theme application
- Add early boot theme script to prevent FOUC
- Support system preference with live updates

src/components/Settings.tsx

@@ -4,6 +4,7 @@ import { RelayPool } from 'applesauce-relay'
 import { UserSettings } from '../services/settingsService'
 import IconButton from './IconButton'
 import { loadFont } from '../utils/fontLoader'
+import ThemeSettings from './Settings/ThemeSettings'
 import ReadingDisplaySettings from './Settings/ReadingDisplaySettings'
 import LayoutNavigationSettings from './Settings/LayoutNavigationSettings'
 import StartupPreferencesSettings from './Settings/StartupPreferencesSettings'
@@ -159,6 +160,7 @@ const Settings: React.FC<SettingsProps> = ({ settings, onSave, onClose, relayPoo
       </div>
 
       <div className="settings-content">
+        <ThemeSettings settings={localSettings} onUpdate={handleUpdate} />
         <ReadingDisplaySettings settings={localSettings} onUpdate={handleUpdate} />
         <LayoutNavigationSettings settings={localSettings} onUpdate={handleUpdate} />
         <StartupPreferencesSettings settings={localSettings} onUpdate={handleUpdate} />

src/components/Settings/ThemeSettings.tsx

@@ -0,0 +1,49 @@
+import React from 'react'
+import { faSun, faMoon, faDesktop } from '@fortawesome/free-solid-svg-icons'
+import { UserSettings } from '../../services/settingsService'
+import IconButton from '../IconButton'
+
+interface ThemeSettingsProps {
+  settings: UserSettings
+  onUpdate: (updates: Partial<UserSettings>) => void
+}
+
+const ThemeSettings: React.FC<ThemeSettingsProps> = ({ settings, onUpdate }) => {
+  const currentTheme = settings.theme ?? 'system'
+
+  return (
+    <div className="settings-section">
+      <h3 className="section-title">Theme</h3>
+      
+      <div className="setting-group setting-inline">
+        <label>Appearance</label>
+        <div className="setting-buttons">
+          <IconButton 
+            icon={faSun} 
+            onClick={() => onUpdate({ theme: 'light' })} 
+            title="Light theme" 
+            ariaLabel="Light theme" 
+            variant={currentTheme === 'light' ? 'primary' : 'ghost'} 
+          />
+          <IconButton 
+            icon={faMoon} 
+            onClick={() => onUpdate({ theme: 'dark' })} 
+            title="Dark theme" 
+            ariaLabel="Dark theme" 
+            variant={currentTheme === 'dark' ? 'primary' : 'ghost'} 
+          />
+          <IconButton 
+            icon={faDesktop} 
+            onClick={() => onUpdate({ theme: 'system' })} 
+            title="Use system preference" 
+            ariaLabel="Use system preference" 
+            variant={currentTheme === 'system' ? 'primary' : 'ghost'} 
+          />
+        </div>
+      </div>
+    </div>
+  )
+}
+
+export default ThemeSettings
+