[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?