If you are experiencing this issue, the most likely culprit is the dynamic.css file not being generated.
The reasons for this could be any of the following:
⋅ A － A security plugin, or perhaps some other plugin - please try to deactivate all your third party plugins, except plugins that came bundled with the theme and see if that helps. If the issue goes away, it would be best to activate each of your plugins one by one and see if you can locate the one that is causing the issue.
⋅ B － Permissions on JS and CSS folders - to alter these, you need to FTP into your hosting and set permissions to 755 on theme_folder/css and theme_folder/JS folders or it could be structured like here theme_folder/assets/css and theme_folder/assets/js. Permissions for all PHP files in these folders should be 755, too. Perhaps you could contact your hosting to do that for you.
⋅ C － PHP settings on the server – to tackle this issue, you can try different PHP version (anything above 5.6). In addition, you should contact your hosting to check why the PHP scripts are not being executed, and files are not being generated.
⋅ D － You can try adding this to wp-config.php:
define('FS_METHOD', 'direct');