Fix several CSS issues, make things a bit tidier

- Adds a custom stylesheet to allow edits per-board
- Fixes size of Octocat causing it to overflow
- Fixes CSS positioning of background icons to something more sane
- Removes custom positioning declaration and uses simple X/Y positioning

![1366 x 768](http://cl.ly/image/0p3B081c3s0p/1366x768%2B0%2B0.png)
![1440 x 900](http://cl.ly/image/09410O3V2m0k/1440x900%2B0%2B0.png)
![1824 x 984](http://cl.ly/image/3I1s1f1m3A1J/1824x984%2B0%2B0.png)