Wrong paths in CSS for relative images

Issue Summary

I used Zenophile to create a subtheme for my site and selected the Midnight option. However, I noticed that some of the images weren't displaying. I had a look at the CSS and discovered that the Midnight images have URLs like 'images/menu-expanded.png'. As the CSS files are located in a directory of their own, I simply changed the URLs to '../images/menu-expanded.png' and the images work.

I went through and fixed all the image URLs in the Midnight CSS files and created the following patch.
Note that I haven't actually tested the patch as such (since the patch affects Zenophile and I've already used it to create my subtheme), but a manual change of the URLs in my subtheme's Midnight CSS files works.