Adaptive UI Colors

This PR is supposed to make dark themes usable with Vico. I find the light gray UI elements very distracting when working with a dark theme, especially in full screen mode.

Don't merge this yet. My current plan is to do the same thing to the dividers and maybe even the sidebars.
Would you accept such a change?

**Without adaptive colors:**
![vico-line-numbers-before](https://f.cloud.github.com/assets/6915/2179787/2903b600-96d8-11e3-8e13-a5eb32c159a6.png)

**With adaptive colors:**
![vico-line-numbers-after](https://f.cloud.github.com/assets/6915/2179786/28c993b2-96d8-11e3-808b-d80461c856c9.png)