Disclaimer

This is not an excuse to bully or deride requests for communication and progress on accessibility needs. Do not tone police people asking for these things, please.

Dark Mode and Accessibility

"Dark Mode" in specific is not something covered by most accessibility guidelines directly, because there is very much no one-size-fits-all design criteria. In general, you should be making it possible for users to personalize the styling of your website (or software) to fit their needs. As long as you are not blocking users from being able to make those changes, you have done your duty from a legal and international standard perspective re: Dark Mode.

Of note: providing your own dark mode can actually be an issue, because you are now responsible for making it work with all color-related success criteria in addition to whatever the default theme for your site is.