Enhance styling system with new themes and fix modal theming consistency

- Add 4 new dark themes: Midnight, Charcoal, Nord, Dracula with complete CSS variable definitions
- Create Apple Calendar style with glassmorphism effects and theme-aware design
- Fix Google Calendar style to be theme-aware instead of using hardcoded colors
- Replace hardcoded colors in modal CSS with theme variables for consistent theming
- Add data-style attribute support to document root for style-specific CSS selectors
- Update sidebar dropdowns to include all new theme and style options

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

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

frontend/index.html

@@ -8,6 +8,7 @@
     <link data-trunk rel="css" href="styles.css">
     <link data-trunk rel="css" href="print-preview.css">
     <link data-trunk rel="copy-file" href="styles/google.css">
+    <link data-trunk rel="copy-file" href="styles/apple.css">
     <link data-trunk rel="copy-file" href="service-worker.js">
     <link data-trunk rel="icon" href="favicon.ico">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" integrity="sha512-DTOQO9RWCH3ppGqcWaEA1BIZOC6xxalwEsw9c2QQeAIftl+Vegovlnee1c9QX4TctnWMn13TZye+giMm8e2LwA==" crossorigin="anonymous" referrerpolicy="no-referrer" />