Google is finally rolling out a system-wide dark mode in Android Q, and the community is wasting no time making it better. The new app DarQ from Quinny899 on XDA Forums lets you force dark mode for the apps of your choosing. The apps you don't want darkened will continue rendering normally. However, you'll need root to play around with DarQ.
Android Q includes an option to force dark mode on, but this applies to all apps. It's a useful way for developers to test dark modes, but a lot of apps don't take kindly to dark mode right now. DarQ can force dark mode for select apps to make sure everything keeps working and you don't burn your retinas quite so much. DarQ can toggle on dark mode in your selected apps all the time or only at night. That's an entirely new feature that isn't part of Android Q, but Google has played around with time-based toggles in past Android betas.
You need Android Q on a rootable device to use DarQ—the latest Magisk works on all Pixel 3 and 3a devices, so that's probably your best bet. The app isn't in the Play Store, so you'll have to grab it from XDA.