Add dark mode toggle

[prakshithamalla-art]

Task

Implement dark/light theme switching

Requirements

• Toggle button in header
• Save preference to localStorage
• Smooth transition
• Update all CSS variables

[prakshithamalla-art]

@komalharshita Please assign these issues to me under GSSoC'26. I can work on them.

[hansikawork-432]

Hi @komalharshita .

I am a GSSOC'26 contributor and I would like to contribute to this issue.

My contribution would be:

1. To add a toggle button that switches light/ dark theme.

2. To also change the font styling and colors so that during the transition the font is visible to users.

3. To test the changes for both desktop and mobile devices.

I am experienced in HTML and CSS.

Please assign this issue to me under #GSSOC'26.