Updated css classes to match new GitHub names.

Following the CSS change form a few days ago, another one broke diff splitting.

While fixing this I found a bit of code based on a class that does not exists and I don't know what it was meant to do. There is a comment in the code to point it out.
If you'd like me to update that as well I'll be happy to, as soon as I know the original intention :smile: 
Same issue here