Firefox is doing dark mode differently to Chrome, and says it's no passing fad See more Computing news TechRadar newsletter.
It kind of takes away the purpose of having a dark mode. The below is a handy guide on how to turn on dark mode on the desktop version of Brave Browser.
For example you can control the built-in dark themes of: YouTube, Reddit, Twitch, 9gag, DuckDuckGo and many others are included every week.