Add theme customization system and theme manager

Introduces a full browser customization system with support for theme presets, live editing, import/export, and non-destructive design. Adds new documentation for customization, updates the features list, and implements a ThemeManager for loading and managing themes at the application level. Updates home and settings pages to support live theming and preview, and adds four built-in themes (default, forest, ocean, sunset).

themes/ocean.json

@@ -0,0 +1,17 @@
+{
+  "name": "Ocean Depths",
+  "colors": {
+    "bg": "#1a365d",
+    "darkBlue": "#2a4365",
+    "darkPurple": "#2c5282",
+    "primary": "#3182ce",
+    "accent": "#00d9ff",
+    "text": "#e2e8f0"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #1a365d 0%, #2c5282 100%)",
+  "version": "1.0",
+  "description": "A calming ocean-inspired theme with blues and teals"
+}