feat: implement working theme toggle button

Issue Description

Problem

The application currently does not provide a functional theme toggle option for users to switch between light and dark modes.

Expected Behavior

Users should be able to:

• Toggle between light and dark themes
• See UI changes instantly
• Retain selected theme after refreshing the page

Proposed Solution

• Create a theme toggle button component
• Implement theme state management
• Store user theme preference in local storage
• Apply theme styles globally across the application
• Add smooth transition effects for better UX

Acceptance Criteria

• Theme toggle button is visible and functional
• Light and dark themes switch correctly
• Selected theme persists after refresh
• UI remains responsive in both themes
• No layout or styling issues after switching themes