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To load these themes just down load the self extracting file and run it.  It will default extract to c:\Program Files\Yahoo!\Messenger\Skins and then creating it's own folder.  If you have Yahoo! loaded elsewhere you will have to change the extract-to-entry before you do the un-zip. 
Yahoo! Messenger Themes
(Cartoon)
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Download Spiderman Theme
Download ORK Theme
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Download Tweedy Painting Theme
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Download Tweedy Angel Theme
Download Two Penguins Theme
Nature ThemesFanatsy ThemesCorporate Themes  -  People Themes
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Download Winnie the Pooh Theme