The default skin is kind of ugly. I used Christian Kreb's m 7 for a while, which is very clean, and lately I've been using c00.

m 7: http://my.cn.opera.com/community/customize/skins/info/?id=45... (doesn't seem to be on the US site anymore, no idea why)

c00: http://my.opera.com/community/customize/skins/info/?id=8692