I'm trying to use the Zero Dark Matrix DevTools theme in Chrome 114.0.5735.134 (Official Build) (32-bit emulated) and am getting nowhere.
I have searched both Google and this site for answers to this question, but have come up with nothing but seemingly-outdated information that suggests the existence of an "Allow custom UI themes" setting within the DevTools Experiments tab, but no such setting seems to exist in the most-recent stable build (as of the time of writing).
The theme extension's overview page (linked above) also mentions an "Enable Developer Tools experiments" flag within chrome://flags, but that seems to have been fully integrated as a stable feature some time ago.
What gives?! Has Google completely dropped support for custom DevTools themes? MTIA :-)
As suggested by wOxxOm in the comments, the Allow extensions to load custom stylesheets setting within the DevTools Experiments settings is the only thing that now needs to be turned on.
I had already found this by myself several hours before posting, but this is the really crucial step: restart Chrome after enabling this setting!
I can't believe I was so close yet so far away, and all because I was too silly to think of the most obvious thing! Anyway I hope this helps someone else in the future ;-)