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/mocha-coffee.json

@@ -0,0 +1,17 @@
+{
+  "name": "Mocha Coffee",
+  "colors": {
+    "bg": "#3c2414",
+    "darkBlue": "#4a2c1a",
+    "darkPurple": "#5d3a26",
+    "primary": "#d2691e",
+    "accent": "#daa520",
+    "text": "#faf0e6"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #3c2414 0%, #5d3a26 100%)",
+  "version": "1.0",
+  "description": "A warm, cozy theme inspired by coffee and chocolate tones"
+}