Please try the following;
– pick a skin folder (ie Windows7) and copy it and then rename it to Windows7v2.
– rename all css files from *.Windows7.css to *.Windows7v2.css
– do the same for gif files
– using Visual Studio (or any other tool that can do a search and replace within multiple files) search and replace Windows7 with Windows7v2
– do your changes and mark them as "!important"
– clear the browser's cache and restart it
You should be seeing the changes after that.
If you still don't see them please debug your code (using IE standard debug tool) and check the css files loaded and look for your changes.
If you still have any questions/issues please let me know.