ScaleNine offers a list of 10 Ways to Skin a Flex App. There are lots of great links to resources for skinning your app from CSS to programmatic skinning to 9-slice. What a terrific resource this single webpage is for any developers who work with Flex 2. Even if you are interested in only relatively minor customizations such as adding an exotic font to your app or icons which heighten the visual experience of it for users and improve the overall interface of it, you’ll find it in this listing. This would be an appropriate resource to pass around to your colleagues who also create Flex 2 apps, that is exactly what I did.
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Free Themes for Flex
ScaleNine (cool site name by the way) is offering free, high-quality Themes for your Flex 2 apps. There are currently 6 themes in the collection with more coming in the near future. Update: after quite a while I went back to take a look at how actively new skins and themes for Flex 2 are being added and happily I can report that dozens and dozens of absolutely gorgeous and interesting themes are now available on the site.
There is even a provision for  learning how to create your own skins for Flex and AIR right on the sidebar that I think a lot of developers will have an interest in. But again, it will be a long time before you exhaust the possibilities for teriffic themes already available to you on the site.