Hi,
You may want to take a look at the Skinning Overview. To solve this issue, you need to add the code below to your user.config. After adding it to your user.config, you need to open and then save your web.config to restart the application.
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />
HTH,
Joe D.