Add better CSS for guides dropdown

[Topher Fangio]

I've noticed that it's sometimes very hard to see what category a particular guide is in, and I think it should be easier to view at a glance. In addition, having the placeholder guides look identical to the categories is painful.

Anyone mind if I clean this up a bit (I'll just make the changes in my fork and submit a pull request, like always)? Any suggestions if I do?

[Tom Dale]

I don't have any suggestions, but I agree that this is a problem.

[Peter Wagenet]

I think this has been fixed.

[Topher Fangio]

Indeed, I meant to add a "Closes #42" to the commit message and forgot. Sorry about that, this can be closed.