[UX][D8] Denote which theme has been set as the admin theme

[klonos]

We denote which one has been set as the default theme:

We should also denote which one is the admin theme, same as D8/9:

[klonos]

PR up for review: backdrop/backdrop#3166

PS: I've also left a comment about this in the PR: I would like some feedback on whether we should do it, plus if it should be done here or a separate issue. I would like to have the is_default and is_admin properties be available in the theme objects returned by system_rebuild_theme_data() and/or list_themes().

[indigoxela]

Is this still relevant? In a way this seems a duplicate of - or at least it's overlapping with #3617