Fixed dark style for Brackets Sprint 43 dark style.

This PR makes sure that extension rating is ready for Brackets Sprint 43's dark style.

Mostly color tweaks.

Before:
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![screen shot 2014-08-17 at 3 05 01 pm](https://cloud.githubusercontent.com/assets/1495261/3946022/3598946c-265c-11e4-9eb2-84633777b74b.png)

After:
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![screen shot 2014-08-17 at 3 11 24 pm](https://cloud.githubusercontent.com/assets/1495261/3946027/42fc4ab8-265c-11e4-9ad7-d733dd2c890f.png)