The original dark mode guide details how to get a global dark mode, but it requires a GUI tool to be executed in each appvm, which isn’t conducive to easy administration.
To get universal darkmode, just replace this action by appending GTK_THEME=Adwaita:dark to /etc/environment. This sets all apps to use darkmode (Adwaita) from the template. I’m not an expert, so there may be exceptions. but it works on every app I’ve tested. You also want to install the correct packages and other configuration like mentioned in this guide (which I think is better than the previously mentioned one).
This was tested on dom0, Fedora, and Debian templates, and I’ll be adding this to the guides if the mods approve.