gtk-key-theme

There may still be a better way to achieve this, but this is the only
way that worked so far. Though the arch wiki says that system-wide
settings can be placed in
  /etc/gtk-2.0/gktrc
  /etc/gtk-3.0/settings.ini
those locations weren't being picked up, so I'm manually adding
  /etc/xdg
to XDG_CONFIG_DIRS, in which gtk apps will search for settings.

https://github.com/bennofs/etc-nixos/blob/5fe80d256d0101b1e73c9036d0a7ae6cee2c4267/conf/desktop.nix

README.md

@@ -10,7 +10,7 @@ Try some window managers
   - set $mod to super
   - run dmenu (or j4) by pressing $mod alone
   - equivalent gsettings
-    - [ ] desktop.interface gtk-key-theme "Emacs"
+    - [✓] desktop.interface gtk-key-theme "Emacs"
     - [ ] desktop.interface clock-format "12h"
     - [✓] services.xserver.libinput.tapping
     - [✓] services.xserver.libinput.clickMethod

environment/default.nix

@@ -21,6 +21,11 @@ in {
         login ${githubUsername}
         password ${githubToken}
       '';
+      "xdg/gtk-3.0/settings.ini".source = etc/xdg/gtk-3.0/settings.ini;
+      "xdg/gtk-2.0/gtkrc".source = etc/xdg/gtk-2.0/gtkrc;
     };
+    extraInit = ''
+      export XDG_CONFIG_DIRS="/etc/xdg:$XDG_CONFIG_DIRS"
+    '';
   };
 }

environment/etc/xdg/gtk-2.0/gtkrc

@@ -0,0 +1 @@
+gtk-key-theme-name = "Emacs"

environment/etc/xdg/gtk-3.0/settings.ini

@@ -0,0 +1,2 @@
+[Settings]
+gtk-key-theme-name = Emacs