Timer: better dark theme colors

Old:
![screenshot - 08182014 - 05 52 04 pm](https://cloud.githubusercontent.com/assets/4411471/3958639/f2404c48-2721-11e4-9e4f-4c7ab2b2d4a7.png)

New:
![screenshot - 08182014 - 05 49 07 pm](https://cloud.githubusercontent.com/assets/4411471/3958643/f8024834-2721-11e4-8a00-8a3320231307.png)
![screenshot - 08182014 - 05 49 19 pm](https://cloud.githubusercontent.com/assets/4411471/3958646/fb16db3e-2721-11e4-9777-e36993bfc2b8.png)
![screenshot - 08182014 - 05 49 30 pm](https://cloud.githubusercontent.com/assets/4411471/3958647/fdf9174a-2721-11e4-8e70-4f1b8f38dc86.png)
(the weird dotted line in the last one is from the screenshot program)

cc @chrismorast