Darkening text on selected tab in light mode

Resolving issue #1

https://github.com/gcmt/plum/issues/1

New colors:

![screen shot 2013-09-22 at 1 52 18 pm](https://f.cloud.github.com/assets/643417/1188031/bcee05b0-23af-11e3-9776-d30aa493b75b.png)