Hi Jason,
Not here (also from my design page). I think this is the way to select the test-blog if it is existing.
As I wrote, I found an older post from 2019 with a description how to create the test-blog. But I cannot find this button.
In an article on the Posting API there is a similar description (Design → Edit Custom Themes → New Test Blog) which also doesn‘t work for me (I cannot find the button).
Regards
Thomas