Add theme toggle feature with light and dark mode support

[devloai]

Description

This PR implements a theme toggle feature that allows users to switch between light and dark modes in the Space Race game.

Changes

• Added CSS custom properties for theme colors (dark and light)
• Created a theme toggle button with sun/moon icons in the top-right corner
• Implemented JavaScript for theme switching and persistence using localStorage
• Updated UI elements to use theme variables instead of hardcoded colors
• Added smooth transitions for theme switching
• Updated README.md with theme toggle feature information

Testing

• Verified theme toggle works correctly on both desktop and mobile layouts
• Confirmed theme preference persists across page reloads
• Tested all UI elements in both light and dark modes

Follow-up suggestions

• Add user preference detection based on system settings using prefers-color-scheme media query
• Implement additional theme options beyond just light and dark (e.g., high contrast, sepia)
• Add animations to the theme toggle button when switching between modes