Until recently, you had to jump through hoops to access the feature on desktop via the developer mode. On desktop and mobile, things are a lot simpler now. To access it, you simply head to your account icon, hitting settings, and look for a setting called "Dark theme." You can then toggle it on or off, as shown in the GIF below. The feature is rolling out today, but as usual, it might not hit your neck of the woods right away.