Improved design

Before launching an MVP, I wanted to touch up the UI and fix any remaining bugs I've found.  This is the first part which is just some overall design changes.  I'll have another branch with Firefox-specific fixes, performance adjustments, and another other changes I want to see before launch.

- Improvements to the visual style.
- Adds in feedback from David.
- Fixes issues with notifications only running if the extension page was left open.