Agreed. Discourse is marketed as customizable. The admin section should be themeable.
Default out if the box is fine. But the advantages of being themeable can be good for other platforms’ migrating to discourse with reducing some of the learning curve. As admin themes could be used to mimic other platforms admin interface somewhat.
The side panel is alright but needs options to customize. Ie maybe a favourites section at top for admins to place their often used links.