I sympathise, but they may have reason to do what at first seems insane.
Because of the legacy of Canvas content created in the context of a locked down white page, much of the existing content (ironically including some adjustments made by the inbuilt accessibility checker!) are not prepared to be viewed in dark mode. The problems aren’t confined to simple poor accessibility standards, the reality is that a lot of content is so completely invisible, it won’t even be apparent you are missing things. Dark mode has a knack for revealing how sloppy the code created by the rich text editor and practices like copy-pasting from various sources can be.
Existing pages and content will often need a lot of work to be dark mode ready, and there isn’t yet widespread awareness or understanding of these issues among most of those building the content.
I would guess this might explain why there are blocks in place on the dark mode browser extensions. However, I believe the ‘genie is out of the bottle’ and these strategies to hold back dark mode are doomed to fail. The fact is many students are already using browser plugins to force a dark mode on pages, and Instructure them selves have endorsed dark mode by building it into their own mobile apps! I would argue prematurely, seeing as it is in direct conflict with their approach in the web browser, but here we are!
As a learning designer building content for Canvas, I am presently focused on implementing dark mode ready practices to ensure that we improve upon our past errors in this respect. I’m eager to discuss these strategies with others invested in this idea, so feel free to reach out.