Use specialisation to switch themes on NixOS

hm/style.nix

@@ -3,7 +3,8 @@ let
   # Currently last commit in https://github.com/danth/stylix/pull/194
   stylix = builtins.fetchTarball "https://github.com/willemml/stylix/archive/2ed2b0086b41d582aca26e083c19c0e47c8991e3.tar.gz";
   polarityFile = "${config.xdg.stateHome}/theme_polarity";
-  polarity = if builtins.pathExists polarityFile then lib.strings.fileContents polarityFile else "light";
+  polarityFromFile = if builtins.pathExists polarityFile then lib.strings.fileContents polarityFile else "light";
+  polarity = if config.frogeye.polarity == "dynamic" then polarityFromFile else config.frogeye.polarity;
   phases = [
     { command = "jour"; polarity = "light"; }
     { command = "crepuscule"; polarity = "dark"; }
@@ -53,13 +54,15 @@ in
       runtimeInputs = [ pkgs.brightnessctl ];
       text = (lib.optionalString cfg.enable ''
         brightnessctl set ${builtins.getAttr phase.command cfg}
-        '') + ''
+      '') + ''
         echo ${phase.polarity} > ${polarityFile}
         if command -v home-manager
         then
             home-manager switch
         else
-            sudo nixos-rebuild switch
+            # In two steps to get the visual changes slightly earlier
+            sudo /nix/var/nix/profiles/system/specialisation/${phase.polarity}/bin/switch-to-configuration test
+            sudo /nix/var/nix/profiles/system/specialisation/${phase.polarity}/bin/switch-to-configuration boot
         fi
       '';
     })

options.nix

@@ -3,6 +3,11 @@
   options.frogeye = {
     extra = lib.mkEnableOption "Big software";
     gaming = lib.mkEnableOption "Games";
+    polarity = lib.mkOption {
+      default = "dynamic";
+      description = "Whether to use light theme or dark theme.";
+      type = lib.types.enum [ "dynamic" "light" "dark" ];
+    };
     desktop = {
       xorg = lib.mkEnableOption "Enable X11 support";
       numlock = lib.mkEnableOption "Auto-enable numlock";

os/geoffrey.nix

@@ -1,4 +1,4 @@
-{ pkgs, config, ... }:
+{ pkgs, lib, config, ... }:
 {
   imports = [
     <home-manager/nixos>
@@ -32,6 +32,12 @@
     useGlobalPkgs = true;
   };
 
+  specialisation = {
+    dark.configuration.frogeye.polarity = "dark";
+    light.configuration.frogeye.polarity = "light";
+  };
+
+
   # Because everything is encrypted and I'm the only user, this is fine.
   services.xserver.displayManager.autoLogin.user = "geoffrey";
 }