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>

THEMES.md

@@ -0,0 +1,116 @@
+# Hermes Web UI — Themes
+
+Hermes Web UI supports pluggable color themes. Five themes ship built-in, and
+you can create your own with pure CSS — no Python changes needed.
+
+---
+
+## Switching Themes
+
+**Settings panel:** Click the gear icon, select a theme from the dropdown. The
+preview is instant — the UI updates as you click through options.
+
+**Slash command:** Type `/theme dark` or `/theme light` in the composer.
+
+**Themes persist** across page reloads and server restarts (stored in
+`settings.json` server-side, with `localStorage` for flicker-free loading).
+
+---
+
+## Built-in Themes
+
+| Theme | Description |
+|-------|-------------|
+| **Dark** (default) | Deep navy/indigo with muted blue accents. Easy on the eyes for long sessions. |
+| **Light** | Clean white/gray with dark text. High contrast for bright environments. |
+| **Solarized Dark** | Ethan Schoonover's classic dark palette. Teal background, warm accents. |
+| **Monokai** | Warm dark theme inspired by the Monokai editor scheme. Green/pink accents. |
+| **Nord** | Arctic blue-gray palette from the Nord color system. Calm and minimal. |
+
+---
+
+## Creating a Custom Theme
+
+A theme is a CSS block that overrides the color variables. Add it to
+`static/style.css` (or a separate file that you link after the main stylesheet).
+
+### Step 1: Define your theme block
+
+Every color in the UI comes from these CSS variables:
+
+```css
+:root[data-theme="your-theme-name"] {
+  --bg: #1a1a2e;          /* Main background */
+  --sidebar: #16213e;      /* Sidebar background */
+  --border: rgba(255,255,255,0.08);   /* Subtle borders */
+  --border2: rgba(255,255,255,0.14);  /* Stronger borders */
+  --text: #e8e8f0;         /* Primary text color */
+  --muted: #8888aa;        /* Secondary/muted text */
+  --accent: #e94560;       /* Accent color (errors, warnings, delete) */
+  --blue: #7cb9ff;         /* Primary action color (links, active states) */
+  --gold: #c9a84c;         /* Secondary accent (pinned items, gold highlights) */
+  --code-bg: #0d1117;      /* Code block background */
+}
+```
+
+That's it. Override any or all of these variables. The entire UI adapts
+automatically because every color reference uses `var(--name)`.
+
+### Step 2: Add it to the theme picker (optional)
+
+To make your theme appear in the Settings dropdown, add an `<option>` to the
+theme `<select>` in `static/index.html`:
+
+```html
+<option value="your-theme-name">Your Theme Name</option>
+```
+
+And update the `/theme` command's valid theme list in `static/commands.js`.
+
+### Step 3: Test it
+
+Switch to your theme via `/theme your-theme-name` or the Settings panel.
+Check these areas:
+- Sidebar session list (hover states, active state, project borders)
+- Message bubbles (user vs assistant styling)
+- Code blocks (background contrast, copy button visibility)
+- Tool cards (running indicator, expand/collapse)
+- Settings panel and login page
+- Mobile layout (hamburger sidebar, bottom nav)
+
+### Tips
+
+- **Light themes** need additional scrollbar overrides to avoid dark scrollbars
+  on a light background. See the built-in light theme for the pattern.
+- The **logo gradient** uses `--accent` automatically, so it adapts to your
+  theme without extra work.
+- **Prism.js syntax highlighting** uses its own CDN stylesheet (Tomorrow theme).
+  It works well on dark themes; on light themes the contrast is acceptable but
+  not perfect. Custom Prism theme support is planned for a future update.
+- **No server changes needed.** The `theme` setting in `settings.json` accepts
+  any string — your custom theme name will persist without code changes.
+
+---
+
+## How Themes Work Internally
+
+1. Each theme is a `:root[data-theme="name"]` CSS block that overrides variables.
+2. Switching themes sets `document.documentElement.dataset.theme = name` in JS.
+3. A tiny inline `<script>` in `<head>` reads `localStorage` before the
+   stylesheet loads — this prevents a flash of the wrong theme on page load.
+4. The theme preference is saved server-side via `POST /api/settings` and
+   loaded on boot via `GET /api/settings`.
+5. The `/theme` command and Settings dropdown both update the DOM, localStorage,
+   and server settings simultaneously.
+
+---
+
+## Contributing a Theme
+
+To contribute a new built-in theme:
+
+1. Add your `:root[data-theme="name"]` block to `static/style.css`
+2. Add the `<option>` to the Settings panel in `static/index.html`
+3. Add the theme name to the valid list in `cmdTheme()` in `static/commands.js`
+4. Test on desktop and mobile
+5. Open a PR — themes are pure CSS additions with no backend changes needed

api/config.py

@@ -653,6 +653,7 @@ _SETTINGS_DEFAULTS = {
     'show_token_usage': False,  # show input/output token badge below assistant messages
     'show_cli_sessions': False,  # merge CLI sessions from state.db into the sidebar
     'sync_to_insights': False,  # mirror WebUI token usage to state.db for /insights
+    'theme': 'dark',  # active UI theme name (no enum gate -- allows custom themes)
     'password_hash': None,  # SHA-256 hash; None = auth disabled
 }
 

static/boot.js

@@ -308,7 +308,7 @@ document.querySelectorAll('.suggestion').forEach(btn=>{
 
 (async()=>{
   // Load send key preference
-  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;window._showCliSessions=!!s.show_cli_sessions;}catch(e){window._sendKey='enter';window._showTokenUsage=false;window._showCliSessions=false;}
+  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;window._showCliSessions=!!s.show_cli_sessions;const _theme=s.theme||'dark';document.documentElement.dataset.theme=_theme;localStorage.setItem('hermes-theme',_theme);}catch(e){window._sendKey='enter';window._showTokenUsage=false;window._showCliSessions=false;}
   // Fetch active profile
   try{const p=await api('/api/profile/active');S.activeProfile=p.name||'default';}catch(e){S.activeProfile='default';}
   // Update profile chip label immediately

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;

static/index.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Hermes</title>
+<script>(function(){var t=localStorage.getItem('hermes-theme');if(t&&t!=='dark')document.documentElement.dataset.theme=t;})()</script>
 <link rel="stylesheet" href="/static/style.css">
   <!-- Prism.js syntax highlighting (loaded async, non-blocking) -->
   <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/prismjs@1.29.0/themes/prism-tomorrow.min.css" integrity="sha384-wFjoQjtV1y5jVHbt0p35Ui8aV8GVpEZkyF99OXWqP/eNJDU93D3Ugxkoyh6Y2I4A" crossorigin="anonymous">
@@ -329,6 +330,16 @@
           <option value="ctrl+enter">Ctrl+Enter (Enter for newline)</option>
         </select>
       </div>
+      <div class="settings-field">
+        <label for="settingsTheme">Theme</label>
+        <select id="settingsTheme" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px" onchange="document.documentElement.dataset.theme=this.value;localStorage.setItem('hermes-theme',this.value)">
+          <option value="dark">Dark (default)</option>
+          <option value="light">Light</option>
+          <option value="solarized">Solarized Dark</option>
+          <option value="monokai">Monokai</option>
+          <option value="nord">Nord</option>
+        </select>
+      </div>
       <div class="settings-field">
         <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
           <input type="checkbox" id="settingsShowTokenUsage" style="width:15px;height:15px;accent-color:var(--accent)">

static/panels.js

@@ -958,6 +958,9 @@ async function loadSettingsPanel(){
     // Send key preference
     const sendKeySel=$('settingsSendKey');
     if(sendKeySel) sendKeySel.value=settings.send_key||'enter';
+    // Theme preference
+    const themeSel=$('settingsTheme');
+    if(themeSel) themeSel.value=settings.theme||'dark';
     const showUsageCb=$('settingsShowTokenUsage');
     if(showUsageCb) showUsageCb.checked=!!settings.show_token_usage;
     const showCliCb=$('settingsShowCliSessions');
@@ -992,6 +995,7 @@ async function saveSettings(){
   if(model) body.default_model=model;
   if(workspace) body.default_workspace=workspace;
   if(sendKey) body.send_key=sendKey;
+  body.theme=($('settingsTheme')||{}).value||'dark';
   body.show_token_usage=showTokenUsage;
   body.show_cli_sessions=showCliSessions;
   body.sync_to_insights=!!($('settingsSyncInsights')||{}).checked;

static/style.css

@@ -4,6 +4,29 @@
     --text:#e8e8f0;--muted:#8888aa;--accent:#e94560;--blue:#7cb9ff;--gold:#c9a84c;--code-bg:#0d1117;
     font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",system-ui,sans-serif;font-size:14px;line-height:1.6;
   }
+  /* ── Light theme ── */
+  :root[data-theme="light"]{
+    --bg:#f5f5f7;--sidebar:#e8e8ed;--border:rgba(0,0,0,0.10);--border2:rgba(0,0,0,0.16);
+    --text:#1c1c1e;--muted:#6e6e80;--accent:#c0392b;--blue:#0a6dc2;--gold:#a07a20;--code-bg:#f0f0f5;
+  }
+  :root[data-theme="light"] ::-webkit-scrollbar-thumb{background:rgba(0,0,0,.15);}
+  :root[data-theme="light"] ::-webkit-scrollbar-thumb:hover{background:rgba(0,0,0,.3);}
+  :root[data-theme="light"] ::selection{background:rgba(10,109,194,.2);}
+  /* ── Solarized Dark theme ── */
+  :root[data-theme="solarized"]{
+    --bg:#002b36;--sidebar:#073642;--border:rgba(255,255,255,0.08);--border2:rgba(255,255,255,0.13);
+    --text:#839496;--muted:#657b83;--accent:#dc322f;--blue:#268bd2;--gold:#b58900;--code-bg:#073642;
+  }
+  /* ── Monokai theme ── */
+  :root[data-theme="monokai"]{
+    --bg:#272822;--sidebar:#1e1f1c;--border:rgba(255,255,255,0.07);--border2:rgba(255,255,255,0.12);
+    --text:#f8f8f2;--muted:#75715e;--accent:#f92672;--blue:#66d9e8;--gold:#e6db74;--code-bg:#1e1f1c;
+  }
+  /* ── Nord theme ── */
+  :root[data-theme="nord"]{
+    --bg:#2e3440;--sidebar:#272c36;--border:rgba(255,255,255,0.07);--border2:rgba(255,255,255,0.12);
+    --text:#eceff4;--muted:#9099aa;--accent:#bf616a;--blue:#81a1c1;--gold:#ebcb8b;--code-bg:#272c36;
+  }
   body{background:var(--bg);color:var(--text);height:100vh;height:100dvh;overflow:hidden;display:flex;}
   .layout{display:flex;width:100%;height:100vh;height:100dvh;}
   .sidebar{width:300px;background:var(--sidebar);border-right:1px solid var(--border);display:flex;flex-direction:column;overflow:visible;flex-shrink:0;}

tests/test_sprint26.py

@@ -0,0 +1,109 @@
+"""
+Sprint 26 Tests: pluggable UI themes — settings persistence, theme default,
+custom theme names accepted.
+"""
+import json, urllib.error, urllib.request
+
+BASE = "http://127.0.0.1:8788"
+
+
+def get(path):
+    with urllib.request.urlopen(BASE + path, timeout=10) as r:
+        return json.loads(r.read()), r.status
+
+
+def post(path, body=None):
+    data = json.dumps(body or {}).encode()
+    req = urllib.request.Request(BASE + path, data=data,
+                                headers={"Content-Type": "application/json"})
+    try:
+        with urllib.request.urlopen(req, timeout=10) as r:
+            return json.loads(r.read()), r.status
+    except urllib.error.HTTPError as e:
+        return json.loads(e.read()), e.code
+
+
+# ── Theme settings ───────────────────────────────────────────────────────
+
+def test_settings_default_theme():
+    """Default theme should be 'dark'."""
+    d, status = get("/api/settings")
+    assert status == 200
+    assert d.get("theme") == "dark"
+
+
+def test_settings_set_theme_light():
+    """Setting theme to 'light' should persist and round-trip."""
+    try:
+        d, status = post("/api/settings", {"theme": "light"})
+        assert status == 200
+        d2, _ = get("/api/settings")
+        assert d2.get("theme") == "light"
+    finally:
+        # Reset to dark
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_solarized():
+    """Setting theme to 'solarized' should persist."""
+    try:
+        post("/api/settings", {"theme": "solarized"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "solarized"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_monokai():
+    """Setting theme to 'monokai' should persist."""
+    try:
+        post("/api/settings", {"theme": "monokai"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "monokai"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_set_theme_nord():
+    """Setting theme to 'nord' should persist."""
+    try:
+        post("/api/settings", {"theme": "nord"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "nord"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_settings_custom_theme_accepted():
+    """Custom theme names should be accepted (no enum gate)."""
+    try:
+        d, status = post("/api/settings", {"theme": "my-custom-theme"})
+        assert status == 200
+        d2, _ = get("/api/settings")
+        assert d2.get("theme") == "my-custom-theme"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_theme_does_not_break_other_settings():
+    """Setting theme should not disturb other settings."""
+    d_before, _ = get("/api/settings")
+    send_key_before = d_before.get("send_key")
+    try:
+        post("/api/settings", {"theme": "nord"})
+        d_after, _ = get("/api/settings")
+        assert d_after.get("send_key") == send_key_before
+        assert d_after.get("theme") == "nord"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
+def test_theme_survives_round_trip():
+    """Theme set via POST should appear in subsequent GET."""
+    try:
+        post("/api/settings", {"theme": "monokai"})
+        d, status = get("/api/settings")
+        assert status == 200
+        assert d["theme"] == "monokai"
+    finally:
+        post("/api/settings", {"theme": "dark"})