If your skins are not working or there appears to be no style, the first thing you should do is view the source of the rendered page and copy the url for the csshandler.ashx that you see in the head section. Paste that url in the browser navigation bar.
Do you get an error?
Do you see CSS?
If you are getting an error, you should check the error log.
Possible causes of the symptom where the site has no styles:
The skin does not exist in the correct location on disk
The files exist but the permissions are not correct, lack of read permission means skins will not work. The skin folder are beneath the /Data folder and this folder and all sub folders and files should be assigned as full control for the web process user.