First and foremost, do note that this is a developer option, and it's not perfect. It is also worth noting that you do not need to have a rooted phone in order to make this work.
This option is hidden away in every Android 10 phone, or at least it should be. It is available on both our Google Pixel 3 and OnePlus 7 Pro units.
You can force apps to use dark mode even if they don't have that option
The first thing you'll need to do, is navigate to developer options. If you have enabled developer options, that sub-menu will be available in the "System" sub-menu of the "Settings" app. Depending on your phone, of course.
If you did not enable "Developer options" just yet, don't fret, that is pretty easy to do. Just open up Settings, and scroll down to "About Phone". In that menu, hit the "Build number" section a number of times, until a toast message pops up, saying that you're now a developer.
Before we proceed, do note that developer options are not for everyone. You can really mess things up if you don't know what you're doing, so don't go in there and flip random switches you don't know anything about.
If you're ready, enter the developer options sub-menu. In the top-right corner, you will see a magnifying glass icon. Tap that, and in the search field, type in "override force dark".
Once the option pops up, hit it. You will be navigated to that option in the developer options, at this point, simply flip the switch.
Once you do that, leave developer options. Dark mode should now be available in apps that do not support it by default. You may need to kill a particular app first, before it loads in dark mode. For some apps, it may take a while to activate regardless.
Some apps may glitch out on you
We've tested this in Hangouts, Instagram (stable build, not beta build in which it is available by default), and several other applications. Both Hangouts and Instagram worked perfectly fine, but (Facebook) Messenger did not. This option completely messed up Messenger on our devices, as the majority of the app ended up being white, and blended with the content. That did not happen on my colleague's phone, though… so, it may not happen to you either. As already mentioned, this option is not perfect, which is why it's in developer options.
That may actually happen in other apps as well. So, depending on what apps you use, this trick may be either useful, or frustrating. If you simply have to use an app that doesn't play well with this option, you can simply disable it from the developer options.
We have included some screenshots in the gallery down below, so that you can see how some apps look like follow the activation of this option.