Even if the file permissions were correct, sometimes it can fail to copy all the files from /Data/skins to /Data/Sites/1/skins if that is what happened you can copy them manually.
If all the files do exist at /Data/Sites/1/skins then the next step to troubleshoot the problem is to view the source of the rendered page, copy the url for the csshandler.ashx?... and paste it into the browser url bar.
After doing that do you see CSS or do you get an error.
Hope it helps,