UI design changes

[grjte]

This PR aims to partially address #90 by tidying up the UI a bit with:

• more consistent spacing & text sizes
• a new header and tab navigation that take up less space and open external links in new tabs
• brand colors
• responsiveness (except on the instruction table where this still needs to be fixed)
• backgrounds for the code windows
• sticky playground buttons on scroll

This PR does not include minor UX improvements, which I plan to do separately.

Note: this was a pretty quick pass, so I wasn't very diligent about TypeScript typing - we should revisit the codebase at some point and add proptypes & remove the "any" types.

[grjte]

awesome, looks much better :)

The proof popup is below the header though, everything else looks great!

Thank you! I've left the proof popup for a follow-up PR, since I'm updating its design to a modal.