Release Notes for Safari Technology Preview 71
webkit.org
webkit.org
I’m also happy to see that I can force a certain appearance through the web inspector, since my system appearance varies throughout the day and that means I need to either keep switching it if I want to debug appearance or only work in the light theme during the day.
edit: just tested it on Mojave with gmail and fastmail and it doesn't recognize that the Yubikey has been inserted, so looks like support is not there out-of-the-box
Also, as another comment noted, only CTAP2 is supported for the host to authenticator API, and only the newest Yubico tokens support CTAP2.
Web inspector also has a widget that toggles between light and dark mode.