I was previewing a new theme (always very careful to click Preview, not Activate). As always, the annoying customizer bar was open on the right (very distracting when trying to look at a theme).
In trying to get out of the preview, I clicked something either on the preview itself or on the customizer bar (I suspect the latter) that activated/installed the new theme. Panic!
I reinstalled my original theme, but the activation of the new theme had obliterated hundreds of lines of custom CSS, so it came up badly broken Finding and restoring a previously saved version of my CSS fixed it, but for about 5 minutes I was in heart failure.
When previewing a new theme, there should be nothing -- absolutely nothing -- on the preview screen that could activate that theme. Whatever is on that screen or on the customizer bar now that does so it should be removed or clearly labeled "Activate this theme." Nothing on the customizer bar should be a permanent change when one is in preview mode.
The blog I need help with is piedtype.com.