I refer you to one of many threads here in the Support Community about dark mode:
https://support.khanacademy.org/hc/en-us/community/posts/32992461127309-Dark-Mode 

Summary from 9 months ago: Khan Academy representative wrote "it’s unlikely that a native dark mode will be available in the foreseeable future."
Khan Academy suggests users make use of the free  Dark Reader browser extension

~ Janice, KA user