There are many posts in the Support Community requesting this.  (You can find them by searching on "dark mode".)

Khan Academy's official answer a year ago was that it's unlikely they will have a dark mode in the near future, but they recommend the Dark Reader browser extension (which is free to use).

https://support.khanacademy.org/hc/en-us/community/posts/6283461687437-Dark-mode

Hi! Adding to JaniceHolz -- this blog post might be of interest to you! Seems like dark mode is being experimented with as part of a colour palette and design overhaul. :)

That blog post seems to only refer to the districts classroom KA, hopefully not the main website...