User-defined theme switcher disappears on admin pages

Possibly, but if that is the case, then the option to switch themes should disappear when you navigate to admin, but it doesn’t so I am able to switch themes here. It is only after doing it that it disappears, so it looked to be an omission rather than a safety issue.

yes, and this may be an edge case. I was testing a possible issue with column headers not appearing (or rather being white on white) so wanting to see if the reported issue was theme-related - i.e. some OTHER setting was breaking the admin interface when it should not. So there will be some possible occasions where you might want or need to, but one could keep switching out of admin to do this, but since the option appears here, others may experience the same