Improve Studio "Themes" display

[turolopezsanabria]

Enable the possibility to customise the Demos per Theme:
For instance, Car Factory is almost always dark, and it's really difficult to see with the demo always in white.

Idea 1: Each theme can customise it's BG
Idea 2: Studio global switch dark - light mode

Inspiration
Vercel Design has a switch to light and dark mode:
https://vercel.com/design