I was able to make Khan Academy into Dark Mode without adding extensions to my browser (Edge).
Here's how I did it:
Dark mode on Bing in MS Edge. Not just in Bing but in just about any website you visit, all will be in dark mode. Go to edge://flags and change "Auto dark mode for web contents" to Enabled and you are good to go. For chrome, it will be chrome://flags.
Happy Learning (: