We didn't have a lot of custom CSS, but what we did have was broken by the layout redesign. I haven't really touched the CSS since the redesign but I've been doing a couple of basic stylesheets over the past few days and noticed some... quirks, that might be irritating for people trying to make stylesheets.
A couple of things may need to be rearranged to make user edits less painful, and I imagine things may change again with a mobile redesign, but I can throw something together so people at least know how to do basic stuff.
If you are already going to make one, then yes I'd love to have a guide. Don't go out of your way though if its too much.