CSS Modifications not showing up in preview or when uploaded

Expand your Files Panel in DW to show Remote server and Local site folder.  See screenshot.

Check the modified date & time stamp on your CSS files to ensure they are the same on both sides.

Maybe you are looking at a cached version of the page in the browser? This a common issue, reloading the browser a few times or emptying the cache usually works or view the page in a different browser. If your css is still not being recognised then you know its not a browser issue, but something else.