User cannot switch theme to "system-default" after setting it once

[wagner-intevation]

If a user has once set a custom theme, there is no way for the user to revert to the system's default theme.

Maybe add a new radio button option "System default", which is also the default selection?

[UlrichB22]

I don't see the problem. You can easily switch back to topside. Would it be helpful to just add "(default)" after the configured theme?

[UlrichB22]

For me the problem is not clear.

1. Is is technically not possible for you to change the theme once again?
2. Is not clear what the default theme was before?

According to (1) I had a similar issue when I had invalid subscriptions, see issue #1577. In this case, it is not possible to change anything in the profile.

Please add some more details.

[wagner-intevation]

By default, each user uses the theme set as system default. The, a user can select a different one. Afterwards, the user cannot switch back to the previous state and must always select one. Even if the user selects the theme, which is currently the default, it is set explicitly for the user. If the admin changes the theme, it does not apply for this user.