Add Multiple Theme Options

Description:
Currently, the stopwatch supports only dark and light themes. Let's enhance the user experience by adding multiple beautiful color themes that users can choose from.

Proposed Themes:

Ocean Blue (deep blues with turquoise accents)
Forest Green (nature-inspired greens)
Sunset Orange (warm oranges and pinks)
Purple Dream (lavender and purple tones)
Minimalist Gray (clean grays and whites)
Requirements:

Add a theme selector dropdown in the navbar
Each theme should maintain the glassmorphism aesthetic
Themes should be persistent (saved to localStorage)
Smooth transitions between theme changes
Ensure all UI elements (buttons, text, backgrounds) adapt properly
Files to modify:

[style.css](vscode-file://vscode-app/c:/Users/Gupta/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) - Add CSS custom properties for each theme
[script.js](vscode-file://vscode-app/c:/Users/Gupta/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) - Add theme switching logic and localStorage persistence
[index.html](vscode-file://vscode-app/c:/Users/Gupta/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) - Add theme selector UI
Acceptance Criteria:

5+ beautiful themes implemented
Theme selector accessible in navbar
Themes persist across sessions
All existing features work in all themes
Smooth theme transitions