temporary theme switcher

Temporary theme switcher requested by @fperez. 

(commit ment to nof end in the final PR, so sloppy)

Make cursor gray 50%, because it is painful to change dynamically.
Also set font/fontsize identical to codemirror.

I still see a 1.5 pixel gap between each lines.

![capture d ecran 2014-10-15 a 13 18 21](https://cloud.githubusercontent.com/assets/335567/4644575/479a23d6-545d-11e4-8128-f01f34d04b37.png)
![capture d ecran 2014-10-15 a 13 18 28](https://cloud.githubusercontent.com/assets/335567/4644576/47a61b1e-545d-11e4-9ac3-83539a444d85.png)


You pass this in and then hardcode it inside the function - one of them is redundant.