Add dark mode support to Python app

- Implement CSS custom properties for theming
- Add comprehensive light and dark color schemes
- Create theme toggle button in header
- Add theme persistence with localStorage
- Support system color scheme preference
- Smooth transitions between themes

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

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

static/app.js

@@ -1,4 +1,42 @@
 (() => {
+  // Theme management
+  const themeToggle = document.getElementById("themeToggle");
+  const prefersDark = window.matchMedia("(prefers-color-scheme: dark)");
+
+  function getTheme() {
+    const saved = localStorage.getItem("theme");
+    if (saved) return saved;
+    return prefersDark.matches ? "dark" : "light";
+  }
+
+  function setTheme(theme) {
+    document.documentElement.setAttribute("data-theme", theme);
+    localStorage.setItem("theme", theme);
+    if (themeToggle) {
+      themeToggle.textContent = theme === "dark" ? "☀️ Light Mode" : "🌙 Dark Mode";
+    }
+  }
+
+  function toggleTheme() {
+    const current = document.documentElement.getAttribute("data-theme") || "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("theme")) {
+      setTheme(e.matches ? "dark" : "light");
+    }
+  });
+
   let qs = new URLSearchParams(window.location.search);
   const qInput = document.getElementById("q");
   const channelDropdown = document.getElementById("channelDropdown");