WIP: Visual Styles

This is definitely still a work in progress, but it is looking like _something_.

Outstanding items:

- [ ] Decide on vertical centering (thinking of not doing it)
- [ ] Style selects
- [ ] Add Firefox logo to footer
- [ ] Style error messages (some of this might move to another PR)
- [ ] Determine responsive breakpoints
- [ ] Rework errors on `complete_sign_up` so that the title is correct
- [ ] Confirm linear gradient background parameters