Add dark mode support

[HRmemon]

Fixes #82

Summary

This PR implements dark mode support as requested in #82. It adds a toggle in the header that allows users to switch between light and dark themes.

Features

• 🌓 Dark mode toggle checkbox in the header (next to "Sync scroll")
• 💾 Theme preference saved to localStorage
• 🎨 CSS variables for easy theme customization
• 📝 Full markdown rendering support in dark mode
• 🖥️ Monaco Editor theme switching (vs ↔ vs-dark)

Implementation Details

• Converted hardcoded colors to CSS variables for maintainability
• Dark theme uses #212121 background and #E8E8E8 text for optimal readability
• Integrated GitHub markdown CSS (both light and dark variants)
• Theme preference persists across page reloads using existing localStorage infrastructure
• Minimal changes to existing codebase structure

Visual Changes

• Light mode remains unchanged (default)
• Dark mode provides sufficient contrast as requested
• All markdown elements (tables, code blocks, blockquotes, etc.) properly styled

Testing

• Toggle between light/dark modes
• Theme preference persists after page reload
• All markdown elements render correctly in both themes
• Table borders and styling work in dark mode
• Code blocks display with appropriate syntax highlighting
• Tested on Chrome, Firefox, and Safari

This addresses all the requirements mentioned in the issue:

• ✅ Toggle to switch between light and dark themes
• ✅ Sufficient contrast for readability
• ✅ Accommodates users who prefer dark mode
• ✅ Helps evaluate how content appears in different themes