The ATF test result screenshots might look different than the actual portal page. This can be more often spotted during the testing of service portal.
Release or Environment
Apparently, this is an expected behaviour.
Because, The ATF does not make an actual screenshot but builds the screenshot based on the information available on the page. The process doesn't allow for external resources to be re-loaded
Customers can import the desired CSS/JS as a UI Script and use it locally instead of externally