feat: Sprint 26 — pluggable UI themes (dark, light, solarized, monokai, nord)

Five built-in themes with instant switching, persistent preference,
and zero-flicker loading. Custom themes are pure CSS additions.

Theme system:
- CSS variable overrides via :root[data-theme="name"] blocks
- Flicker prevention: inline <script> reads localStorage before
  stylesheet parses, preventing dark-flash on light-mode users
- Server-side persistence via settings.json (theme field)
- Boot.js syncs server preference to DOM + localStorage

Built-in themes:
- Dark (default): deep navy/indigo, muted blue accents
- Light: clean white/gray, high contrast, scrollbar overrides
- Solarized Dark: teal background, warm accents
- Monokai: warm dark, green/pink accents
- Nord: arctic blue-gray, calm and minimal

UI integration:
- Settings panel: theme dropdown with instant live preview
- /theme slash command: /theme dark|light|solarized|monokai|nord
- No enum constraint on theme setting — custom themes just work

Documentation:
- THEMES.md: how to switch themes, create custom themes, contribute

8 new tests. All 408 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/commands.js

@@ -10,6 +10,7 @@ const COMMANDS=[
   {name:'workspace', desc:'Switch workspace by name',            fn:cmdWorkspace, arg:'name'},
   {name:'new',       desc:'Start a new chat session',            fn:cmdNew},
   {name:'usage',     desc:'Toggle token usage display on/off',   fn:cmdUsage},
+  {name:'theme',     desc:'Switch theme (dark/light/solarized/monokai/nord)', fn:cmdTheme, arg:'name'},
 ];
 
 function parseCommand(text){
@@ -122,6 +123,22 @@ async function cmdUsage(){
   showToast('Token usage '+(next?'on':'off'));
 }
 
+async function cmdTheme(args){
+  const themes=['dark','light','solarized','monokai','nord'];
+  if(!args||!themes.includes(args.toLowerCase())){
+    showToast('Usage: /theme '+themes.join('|'));
+    return;
+  }
+  const t=args.toLowerCase();
+  document.documentElement.dataset.theme=t;
+  localStorage.setItem('hermes-theme',t);
+  try{await api('/api/settings',{method:'POST',body:JSON.stringify({theme:t})});}catch(e){}
+  // Update settings dropdown if panel is open
+  const sel=$('settingsTheme');
+  if(sel)sel.value=t;
+  showToast('Theme: '+t);
+}
+
 // ── Autocomplete dropdown ───────────────────────────────────────────────────
 
 let _cmdSelectedIdx=-1;