fix too intrusive highlight

When writing highlights were too intrusive, I could not even see the text and even less select with the mouse.
I like to work with dark background, maybe in lighter background was working better.
You can see the preview here:
![image](https://cloud.githubusercontent.com/assets/1930175/5062977/66a220c2-6dd4-11e4-8eb2-0cb3cb6c6cc0.png)
![captura de pantalla 2014-11-16 a la s 21 01 34](https://cloud.githubusercontent.com/assets/1930175/5062981/9959dbe0-6dd4-11e4-9b5d-898b3d71e1cd.png)