I was running 2.276, and got:

Dark Theme UNAVAILABLE
UI Themes User Interface
Dark theme
This version of the plugin exists but it is not being offered as an update. This is typically the case when plugin requirements, e.g. a recent version of Jenkins, are not satisfied. See the plugin documentation for information about its requirements.

I upgraded (which restarted jenkins) and returned to the plugin manager, and saw the same general info:

Except, it's now wrong.

Clicking check now fixed it;

The dependency resolution (or whatever) should be reevaluated when the version of jenkins changes.