We have already covered on how to create and share themes on Windows 7. Now if you want to create custom themes for Windows 7, then Style 7 is tool to create custom themes for Windows 7. Style 7 is a Windows 7 theme generator and it uses the default Microsoft Aero theme as base and generates a new theme using blackbox styles. Blackbox styles are created with a single, simple, human-readable, configuration file.
There are numerous such themes available for download. You will need UxStyle Core or UxTheme patcher to run the Unsigned Themes created by Style7. The installation includes few themes, but you can search and download for more themes.
We have not tested the themes created by this tool as it requires patching. Further the software while installation changes the default search engine if the option is not unchecked while installing.
Just wrote a post with a whole lot of free themes for Windows 7. Read it here:
http://ninjarabbits.blogspot.com/2009/11/got-windows-7-get-some-free-themes.html