Add Dark Mode Support

[dakshmehta007]

Description

Added dark mode functionality to improve user experience and reduce eye strain during nighttime usage. The implementation includes a theme toggle button and consistent dark theme styling across components.

Changes Made

• Created theme context for managing dark/light mode state
• Added theme toggle button in the top-right corner
• Implemented dark mode styles for the main app and ScrollToTop component
• Added smooth transitions for theme changes
• Ensured theme persistence during scrolling

Screenshots

Testing Instructions

1. Run the app locally
2. Click the theme toggle button (🌙/☀️) in the top-right corner
3. Verify that:
   Background and text colors change appropriately
   The ScrollToTop button adapts to the current theme
   The Theme toggle works consistently
   Transitions are smooth
   Components remain functional in both themes

Technical Details

• Uses React Context API for theme management
• Implements CSS variables for consistent theming
• Maintains accessibility standards in both modes

Related Issue

Closes #2: Dark mode implementation Request