Add pseudo-dark mode while maintaining XP aesthetic

- Create CSS variables for theme customization
- Define light and dark XP color schemes
- Dark mode uses darker backgrounds, lighter text
- Maintain classic XP beveled borders and shadows
- Add moon/sun toggle button in title bar
- Implement theme switching with localStorage
- Support system color scheme preference
- Smooth transitions between themes

The dark mode keeps the Windows XP look with:
- Dark teal/navy desktop background
- Dark gray window and button faces
- Lighter text colors
- Darker gradient title bars
- All original XP styling preserved

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

static/app.js

@@ -1,4 +1,45 @@
 (() => {
+  // Theme management
+  const themeToggle = document.getElementById("themeToggle");
+  const prefersDark = window.matchMedia("(prefers-color-scheme: dark)");
+
+  function getTheme() {
+    const saved = localStorage.getItem("xp-theme");
+    if (saved) return saved;
+    return prefersDark.matches ? "dark" : "light";
+  }
+
+  function setTheme(theme) {
+    if (theme === "dark") {
+      document.documentElement.setAttribute("data-theme", "dark");
+      if (themeToggle) themeToggle.textContent = "☀️";
+    } else {
+      document.documentElement.removeAttribute("data-theme");
+      if (themeToggle) themeToggle.textContent = "🌙";
+    }
+    localStorage.setItem("xp-theme", theme);
+  }
+
+  function toggleTheme() {
+    const current = document.documentElement.getAttribute("data-theme") === "dark" ? "dark" : "light";
+    setTheme(current === "dark" ? "light" : "dark");
+  }
+
+  // Initialize theme
+  setTheme(getTheme());
+
+  // Listen for theme toggle
+  if (themeToggle) {
+    themeToggle.addEventListener("click", toggleTheme);
+  }
+
+  // Listen for system theme changes
+  prefersDark.addEventListener("change", (e) => {
+    if (!localStorage.getItem("xp-theme")) {
+      setTheme(e.matches ? "dark" : "light");
+    }
+  });
+
   let qs = new URLSearchParams(window.location.search);
   const qInput = document.getElementById("q");
   const channelDropdown = document.getElementById("channelDropdown");