playground: style simulator like a Bootstrap card

[aykevl]

In my opinion, this looks much better than the previous VSCode-like style.

• It supports better keyboard navigation of the UI (not complete, but better than it was before).
• It now supports dark mode (light+dark), instead of staying dark all the time.
• It integrates with the look&feel of the rest of the website.

This is what it now looks like, which I think is a lot cleaner:

[deadprogram]

For sure looks a lot better, thank you @aykevl now merging.