When you choose a new skin it changes the design of the headers of your contact list as well as your IM conversation window (and adds some accents in other areas). There are currently 7 skins to choose from in Yahoo! Messenger 9.0.
To change your skin in Yahoo! Messenger 9.0, click the gold dot up near the top of the contact list window. It’s located near the minimize / mazimize / close buttons. Once the skin menu appears, hover your mouse on a skin to view it, then click to select it.
via Yahoo Messenger Blog
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