Makes invisible characters and wrap guide darker

I found the invisible character and wrap guide way too bright and distracting, this darkens them so text stands out better.

Before
![before](https://cloud.githubusercontent.com/assets/244198/3095983/8fe7f9fe-e5ce-11e3-925d-4fcf77d73bed.png)

After
![after](https://cloud.githubusercontent.com/assets/244198/3095984/916d51de-e5ce-11e3-9652-b29017c589cc.png)