Walkthrough UI

Sketch of an experimental walkthrough UI. Most of the code is refactoring DOM so that the entire walkthrough is implemented as a CSS :target + sibling system. Only for style editor atm, source editor needs more work before it can take a walkthrough I think.

### Remaining

- [ ] design review @samanpwbb 
- [ ] localStorage or other flag for triggering walkthrough on first run
- [ ] copy/content/angle review