When you have a couple of themes in your installation (in my case I got the Caule-pack running) and you select any theme other then “Backend-selected” you won’t be able to switch back to “Backend-selected” since this option is always the first entry in the list and probably hidden behind the title bar.
Only resetting the app to defaults fixes the issue.
Tested on a Pixel 5, Android 11, Home Assistant Companion 2021.6.2-full.
Yes just tried it, same result.
The first entry of the dropdown is always hidden behind the title bar.
EDIT: Tested around with it a little more, this actually only happens when the Theme-Input is in the lower half of the screen, since then the dropdown opens from the Theme-Input upwards.
When scrolling down a little, so the Theme-Input is in the top half of the screen, the dropdown will open from the Theme-Input downwards and I can select every entry.