There is an existing feature request for a dark mode found here. At the moment, the feature has been deployed to mobile apps but I believe it has not been implemented for browsers. With the recent changes to how features are submitted, this was marked as "Open" earlier in November. The current plans are to resume the ability to vote on ideas in December once they have systems set up. I believe the ability to add comments is coming back as well. Check out more information on the changes coming on Ruth's blog post.
In the meantime, you might check that idea thread linked at the top of my comment to see if anyone suggests any potential work arounds for you to try.