Er, this was fun... Merlin asking on IRC where the darn Admin theme can be set. me answering it's on Themes page. With that knowledge, it takes three minutes to find it. Got no proposal right now, but file this issue so I don't forget.
This collapsible fieldset hides it better than a gray cat in the night. Riddle-Image for the patient attached...

| Comment | File | Size | Author |
|---|---|---|---|
| Screenshot-070.jpg | 82.97 KB | eigentor |
Comments
Comment #1
merlinofchaos commentedIt seems like having the setting right on the 'dashboard' would be a cool place to have it.
Comment #2
eigentor commentedThe position on the theme page appears the right place.
It only needs some formatting which jumps more to the eye and, ideally, is self-explaining (hm a graphic that symbolizes "Admin-Theme"?
Fitting into the discussion here http://drupal.org/node/491214 and given the fact that Drupal 7 will ship with a default Admin theme, things might be different again. But only if the theme selector makes it into D7. So maybe a solution that builds on the current page is enough.
Comment #3
Bojhan commentedLet's move this discussion in the redesigning the themes page, perhaps?
Comment #4
Bojhan commentedI am going to mark this by Design its not a bug, and that its well hidden is by prupose - the redesign of the theme page and introduction of Seven should make it appearant enough for this issue to dissapear.
Comment #5
merlinofchaos commentedSure, and for upgrading sites that use Garland, they'll never actually get to see Seven, nor will they be able to find the option. Good idea.
Comment #6
dddave commentedJust to rant: I spent about 20 minutes to find these settings because I didn't scroll completely down on the appearance page. I am using Aquia Marina/Fusion theme which clutters the disabled themes section which actively discourages scrolling to the bottom.