Did you check the actual custom.css in your server? do you have shell access? How did you edit the file? If the file is actually updated in your server and still getting old version in the browser, some cache mechanism is taking place, somewhere.
Try incognito mode or even a never-used browser. Discard it is related browser cache. Set Prestashop Cache setting to "Force Compilation". Ask your hosting provider if they don't have some auto cache system or a CDN.