If you have made changes in the design settings or added custom CSS and the changes are present when you are working in the visual builder, but get removed as soon as you exit the visual builder, then there could be a static CSS file generation problem.
To confirm, if this is the issue due to static CSS file go to Divi > Theme Options > Builder > Advanced in the Divi Theme Options.
First, clear the cache using the black colored Clear button and then disable the option for Static CSS File Generation as seen in the screenshot below:
If disabling the option fixes the problem, then it was a problem with Static CSS File Generation.
Root Cause of this issue: Most probable reason for failed static CSS file generation is syntax issues in your Custom CSS area. You can copy your whole custom CSS to this URL https://jigsaw.w3.org/css-validator/#validate_by_input to Validate and fix the syntax issue, once all the CSS has been fixed the static CSS file generation should then work.
Note: 1) Sometimes even after disabling this option, the issue might not get fixed, in that case make sure the cached CSS file is deleted from hosting file manager, some web hosts need this file to be manually deleted.
2) If there are no issues in custom CSS code and you are still seeing this problem, you can try contacting your web host to see if they are blocking static CSS file generation. You can also leave the option disabled as well.
3) In any case (if disabling this option fixes the issue or not), once this test is done, you should enable back the option as it helps in loading the CSS changes a bit faster. Disabling this option is needed temporarily just to confirm if this is the static css generation failure issue.
4) If the above solutions don't work, You can also go to Divi > Support Center and here you can enable the safe mode and then take a look, just to rule out if any plugin conflict or any custom code is creating the trouble.