Improve visuals of sub tabs in creator panel

[manuhabitela]

This is a task taken from the right panel tabs issue that should be worked on independently.

The idea is to improve the visuals of subtabs, as it feels better to have inactive tabs not being colored with the primary color:

@lusebille mockups: https://www.figma.com/design/wcpetFt6aOKzTszcvPPWLQ/%5B05%2F24%5D-Grist-Design?node-id=2761-141496&t=qX4iHFYd8uzsW8kZ-1

[manuhabitela]

Actually, working on the new colors first is required here, if I use the current grays they will not be visible, and if I use the new grays just in this issue it'll feel weird to have just those things having colors different from the main theme…

Waiting for feedback on #1295 first.