The font settings at the URL opera://settings/fonts are just for web pages that don't set a font-family for something. They do not override.
You could install https://chrome.google.com/webstore/detail/user-javascript-and-css/nbhcbdghjpllgmfilhnhkllmkecfmpld/related for example and add a new rule that matches https://*/* for example with this CSS:
{font-family: Georgia !important;
}
See https://developer.mozilla.org/en-US/docs/Web/CSS/font-family.
There's an options button for a rule in the extension to control the priority and turn on programmable injection. The documentation page for the extensions shows what all that does.
The extension won't affect Opera's UI and won't affect opera:// pages like settings and flags etc. But, it's something.