Books are judged by their covers. It's a sad truth to those that aren't willing to put their time into a wiki, but the advantage of those whom are. What am I saying? The moral in this story is that an attractive layout is the key to success.
Now lets say that your wiki is on an very wide topic. For example Wikipedia, whoms topic is everything. Their nice, clean skin reflects their openness to any subject. A skin looks good when it is fitting. If you wanted to mix it up a little, the most random example that I could find on Wikia would be the Plastic Cameras Wiki. They didn't chose a clean slate, but what they did is somewhat appealing. Their background image was fitting, and their color scheme was somewhat sepia-based. I can't see why they were as specific as to document plastic cameras only, but what can I say?
Now believe it or not, the attractive image that keeps catching your eye at the top of this page isn't a wiki on the color green. It's the Communipedia, the wiki-styled social network. No, it's not about social networks, silly, it is one! In the rare case that you want your wiki to stand out as an innovative site on it's own, you may chose your own color scheme.
Now as I had hinted earlier, there is more to a skin than coding, but the layout of each page. An attractive page will fit both the skin, as well as the subject. That means that templates and everything, especially the main page, should match very well with the background, while the slate that the writing appears on should by clearly distinguishable from the text itself. You don't want to be held responsible for compromised vision amongst your contributers. Make sure that every aspect of the page appears to fit into the color scheme.
I'd also recommend three best practices, that when combined together, work miracles. Wikia's with bright, single-color based themes, a white, clean slate for reading from (maybe with a white/gray gradient), and dark gray or black text looks great. Combine this with rounded corners on nearly every module, text box, template, search bar (partly, I'll elaborate) and the page itself. When the search bar is rounded, it often looks good, but do to limits in Oasis, i'd only recommend that the left end gets rounded, while the search button itself remains rectangular.