Add new built-in themes and update theme selection UI

Introduces seven new built-in themes (cyberpunk, midnight-rose, arctic-ice, cherry-blossom, cosmic-purple, emerald-dream, mocha-coffee, lavender-fields) with corresponding JSON files, updates the FEATURES documentation, and enhances the settings UI to display and select these themes. Also updates theme loading logic in theme-manager.js and improves the theme selector layout for better usability.

themes/arctic-ice.json

@@ -0,0 +1,17 @@
+{
+  "name": "Arctic Ice",
+  "colors": {
+    "bg": "#f0f8ff",
+    "darkBlue": "#e6f3ff",
+    "darkPurple": "#d1e7ff",
+    "primary": "#4169e1",
+    "accent": "#87ceeb",
+    "text": "#2f4f4f"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #f0f8ff 0%, #d1e7ff 100%)",
+  "version": "1.0",
+  "description": "A clean, cool theme inspired by arctic ice with crisp blue tones"
+}