Implement dark mode

[asthabh23]

Description

Add dark mode support to improve user experience and reduce eye strain in low-light environments.

Requirements

• Add dark mode toggle in site header
• Implement CSS variables for theme colors
• Persist user preference in localStorage
• Respect system preference (prefers-color-scheme)
• Ensure all components support dark theme

Acceptance Criteria

• Dark mode toggle is visible and functional
• All text remains readable in dark mode
• Images and media display correctly
• User preference persists across sessions
• Smooth transition between themes