[PP-1] Refactor Project Browser's styling so that it uses CSS variables to control colors and other Gin-specific switches

Thanks for opening this

As #3505700: Move the Project Browser-specific styling from Gin is in so we can start working on this.Marking it active.

I'll take this up.

I have removed most of the #project-browser usage.The remaining are hard to remove because they are direct child of #project-browser.
Attaching the SS for the same which shows the HTML structure for this.

@saschaeggi do you have any better way to handle the remaining stuff?

Is our compatibility with Gin all sewn up and this can be closed? Or is this work still needed?

I think the code changes look fine, but I'm not really qualified to sign off on them.