I tried changing fonts of the text element by adding a custom css in
Theme Options > CSS in my test sites and it works without issues.
Since the issue in your site can be found in the backend, would you mind providing us your wordpress admin login in
Secure Note so we can take a closer look.
For the meantime, you could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Let us know how it goes!