tailscale-systray: make theme configurable

modules/services/tailscale-systray.nix

@@ -15,6 +15,20 @@ in
     enable = lib.mkEnableOption "Official Tailscale systray application for Linux";
 
     package = lib.mkPackageOption pkgs "tailscale" { };
+
+    theme = lib.mkOption {
+      type = lib.types.nullOr (
+        lib.types.enum [
+          "dark"
+          "dark:nobg"
+          "light"
+          "light:nobg"
+        ]
+      );
+      default = null;
+      example = "dark:nobg";
+      description = "Color theme to use for the Tailscale icon.";
+    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -26,6 +40,12 @@ in
           Tailscale systray is available since version 1.88.1
         '';
       }
+      {
+        assertion = cfg.theme != null -> lib.versionAtLeast cfg.package.version "1.98";
+        message = ''
+          Tailscale systray supports themes since version 1.98
+        '';
+      }
     ];
 
     systemd.user.services.tailscale-systray = {
@@ -41,7 +61,12 @@ in
       Install = {
         WantedBy = [ "graphical-session.target" ];
       };
-      Service.ExecStart = "${lib.getExe cfg.package} systray";
+      Service.ExecStart = lib.concatStringsSep " " (
+        [
+          "${lib.getExe cfg.package} systray"
+        ]
+        ++ lib.optional (cfg.theme != null) "--theme ${cfg.theme}"
+      );
     };
   };
 }

tests/modules/services/tailscale-systray/default.nix

@@ -2,4 +2,5 @@
 
 lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
   tailscale-systray-basic = ./basic.nix;
+  tailscale-systray-theme = ./theme.nix;
 }

tests/modules/services/tailscale-systray/theme.nix

@@ -0,0 +1,19 @@
+{ config, ... }:
+{
+  services.tailscale-systray = {
+    enable = true;
+    theme = "dark:nobg";
+    package = config.lib.test.mkStubPackage {
+      name = "tailscale";
+      version = "1.98.1";
+      outPath = "@tailscale@";
+    };
+  };
+
+  nmt.script = ''
+    serviceFile=home-files/.config/systemd/user/tailscale-systray.service
+    assertFileExists $serviceFile
+    assertFileRegex $serviceFile \
+      '^ExecStart=@tailscale@/bin/tailscale systray --theme dark:nobg$'
+  '';
+}