Add dark mode support

[coder-contrib-bot]

Description

Add a dark mode theme option to improve accessibility and user experience. Modern applications often provide a dark mode option to reduce eye strain and conserve battery life on OLED displays.

Implementation Details

1. Create a new theme in
2. Add a theme toggle component in the header
3. Implement theme switching logic using CSS variables
4. Store user preference in localStorage
5. Add automatic theme detection based on system preferences using media query

Benefits

• Improved accessibility for users with light sensitivity
• Battery life conservation on OLED displays
• Modern UX that aligns with industry standards
• Better reading experience in low-light environments