.... I know there is cache issue. Now it worked. I disabled all cache on the host, and made sure not to have any plugin for caching.
You may need to clear your browser cache - in most browsers, you can use [Ctrl]+[Shift]+[Del] to bring up a dialog with cache-clearing options.
Can you provide a link to the site that is having issues? I just looked at the demo, and the custom.css file is definitely being called, so I'll need to see what's happening on your site before I can figure out the issue.