On gnome, gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita-dark' can set dark theme system-wide.
However, the same doesn't happen with a sway session. Firefox browser still detects light theme.
Firefox uses the GTK theme settings. On sway I could set the setting gtk-application-prefer-dark-theme to 1 to get Firefox and GTK apps to use a dark theme by default.
You'll need to edit (or create if it doesn't exist) the following file:
~/.config/gtk-3.0/settings.ini
To look something like this:
[Settings]
gtk-application-prefer-dark-theme=1
After that firefox and GTK apps were dark by default. It will likely require a restart of the application in the case of them already running.
From my experience you need to set theme (or rice) manually. there are tools that allow it to be more automated like pywall, but for your purposes, i believe you want to switch the icon and background theme. to do this i personally use lxappearance and qt5ct. I dont personally know how to change all themes of applications with only a window manager though.
If you want a video to help with this here is a good source: https://www.youtube.com/watch?v=x_Mid3_1Uzo
lxappearance works for me on i3 wm with linux mint
lxappearance is a program to change GTK+ themes, icon themes, and fonts used by applications. It is a feature-rich GTK+ theme switcher.