Code: su

Code: settings put secure ui_night_mode 2

Code: reboot

I stumbled across this in the Viper4Android 2.7 thread. It was suggested for activating dark mode for that app, which it does, but I noticed that it also changed the playstore, photos, and chrome (kind of). There may be more but those are the only Google apps that I use beside Drive and messages which already have a dark theme option within the app.Edit- I forgot about Gmail. It doesn't work for that one, at least not on my end.This is only confirmed on Stock 6.0.1 firmware.First you'll need a terminal app. I use Termux Open that app and type the following:Press enter, then type (or copy/paste)Press enter, then reboot or type in the terminalAnd press enterOnce your phone boots back up you should see dark mode on the above mentioned apps and possibly more.Credit to this post by @ dpEnd367