hyperlinks hard to read in dark themes

In dark themes, hyperlinks are hard to read:
![screenshot 2014-09-18 at 10 07 47 am](https://cloud.githubusercontent.com/assets/9863/4324031/56e500b6-3f56-11e4-87c9-85606ccd4135.png)
![screenshot 2014-09-18 at 10 07 35 am](https://cloud.githubusercontent.com/assets/9863/4324033/59dbcc14-3f56-11e4-8e13-cb47a33294ca.png)

Please add styles for `a` elements in the dark themes.

Thanks for your consideration.