I use the website version and there isnt an option for a dark mode and i use a chrome extension to keep my eyes from burning lol but its inverts all the colors and looks weird. I suggest not only implementing a dark mode option but also the color scheme Hytale uses in its gui. Hytale is an upcoming game and it uses a color scheme for the gui that is clean and engaging with its kind of dark blue with a little red and greenish hue too. That would be awesome and make khan academy look incredible. Hytale is a true passion project like Khan Academy and they've done the research about making good gui and have achieved perfection its incredible. I'd say look at their blog post about it and give Khan Academy dark mode and Hytale themes in the settings.

Heres the link to Hytale's gui: https://hytale.com/news/2018/12/a-sneak-peek-at-hytale-s-user-interface

Also add hotkeys that work on the app and website for computers. For examples keys 1-4 can be used to instantly select options a, b, c, or d. Also enter can be used to submit the answers or move on to the next page and things like that.

Other than that Khan Academy is perfect, and huge thanks for all the hard work that literal volunteers put into project like Khan and Hytale. Keep up the incredible work, we all appreciate it more than you know!