Custom theme via prelude-theme variable

The problem with the current approach is that even if we call `disable-theme` explicitly, there still will be some leftovers with the previous loaded theme. In the case of zenburn for example, `smartrep-mode-line-active-bg` get set to "#383838", when you disable zenburn, it's still "#383838", when you load a light theme, it looks like hell.

![screen shot 2014-10-11 at 12 03 59 pm](https://cloud.githubusercontent.com/assets/2468952/4601394/a96e0be6-50fb-11e4-928f-4dd2478d74e0.png)