Improve Studio "Themes" display

[turolopezsanabria]

Discussed in https://github.com/SUI-Components/sui-components/discussions/1505

^{Originally posted by turolopezsanabria May 17, 2021}

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