In case the custom theme icons are missing, you have to follow the troubleshooting steps below:
Step 1 - Update the icons
The first step is to update the icons. Go to Elementor > Custom Icons > theme-icons > hit "Update"
Step 2 - Mixed content error
Does your installation have an SSL certificate? If yes, the missing icons may be due to a mixed content error. You can check this solution Vamtam Elementor Themes: Mixed content SSL warnings
Step 3 - Enable PHP Zip Extension
If you go to Elementor > Custom Icons and you don't see the "Publish" section on the right, then the PHP Zip extension, which provides the ZipArchive class is not available. This is required for Elementor to support custom icon fonts.
You can either enable it from php.ini, if you have access to that and it's already installed on your server, or you can ask your hosting provider to enable it.