This is pretty funny:
I'm guessing this is because none of the themes are identified as the admin theme?
I think the error here is allowing all themes to be disabled. Having zero themes enabled should not be allowed.
Well, they aren't. As you can see, Stark is enabled. It's unclear to me why it's not picking up that since the admin theme is set to the default theme, that Stark should be the admin theme.
This regression seems to be caused by #542828: Do not special case disabled admin theme
We love PHP's loose data typing... ;)
However, we need to update this code anyway for the conversion of these settings to the configuration system. So while being there, I adjusted the default value handling for 'admin_theme'.
The last submitted patch, drupal8.theme-admin.3.patch, failed testing.
It seems to work with this simple modification.
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