Before you go tinkering with Canary just be advised: Canary is Google’s “bleeding edge” Chrome OS path, which receives daily updates of features before they’ve been widely tested. It can only be accessed from Chromebooks switched into a special developer mode (not to be confused with the Chrome OS Developer channel). Google warns that Canary can be “unstable.”
But at the moment, to activate dark mode on your Chromebook, you need to have the Canary channel installed. Once you’ve done that, Android Central says you just open Chrome and type in chrome://flags/#enable-force-dark and chrome://flags/#enable-webui-dark-mode into the URL bar. I should note I tried this on my older Chromebook and wasn’t able to get it to work.