feat: add working theme toggle button

Description

Summary

Implemented a fully working theme toggle button to switch between light and dark modes across the application.

Changes Made

• Added theme toggle button component
• Implemented light/dark theme switching functionality
• Stored selected theme preference
• Updated UI styles for both themes
• Added smooth transition between themes
• Improved user experience and accessibility

Testing

• Verified theme changes correctly on button click
• Checked persistence after page refresh
• Tested responsiveness on different screen sizes

closses #37 (closed)