Improve themed markdown/text/diff views

Themed Diff
==========
![diff-dark](https://cloud.githubusercontent.com/assets/291148/3115067/9ae944be-e701-11e3-8bfb-2f4ff5d4ed09.PNG)
![diff-light](https://cloud.githubusercontent.com/assets/291148/3115069/9aea6a2e-e701-11e3-9ac9-f68d06cb9039.PNG)

Themed Markdown
===============
![markdown-dark](https://cloud.githubusercontent.com/assets/291148/3115068/9aea422e-e701-11e3-8732-ba2c3f2b4bde.PNG)
![markdown-light](https://cloud.githubusercontent.com/assets/291148/3115071/9aebbdfc-e701-11e3-85cd-fc7e44909947.PNG)

Themed Text 
===========
![text-dark](https://cloud.githubusercontent.com/assets/291148/3115070/9aeb9a98-e701-11e3-9f19-a378077e3b39.PNG)
![text-light](https://cloud.githubusercontent.com/assets/291148/3115072/9aec402e-e701-11e3-90fa-bcd3970725ac.PNG)