Good morning, @icehorse ...
This is a pretty popular request here in the Canvas Community, but there hasn't been anything yet implemented by Instructure (the folks that make Canvas). However, if you head on over to this Feature Idea, Dark Theme/Dark Mode for Canvas - Instructure Community (canvaslms.com), you can participate in the conversation. You might want to give that idea a star rating if you feel that it aligns with something you'd like to see Canvas engineers implement in the LMS. You also might want to post a comment letting them know why this would be important to you. (You could even copy/paste your original question here right into that thread.)
I know this might not be the exact answer you were looking for, but I hope it helps in some way to answer your question.
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