[Feature] Add dark/light theme toggle to dashboard

[ASGCompute]

The Unified Control Room dashboard currently has a fixed dark theme. Adding a theme toggle would improve accessibility and user preference support.

The dashboard is built with React + Vite and uses vanilla CSS.

[IlyasFardaouix]

I think I see where the issue might be. The dashboard is built with React + Vite and uses vanilla CSS, but the theme toggle feature is not implemented in the codebase. I'd like to take a look at the CSS files (e.g. src/styles/app.css) to see if we can add a simple toggle class that switches between dark and light themes. Can you provide more context on how you envision the theme toggle working?