deadd-notification-center: Add default css file

The application otherwise crashes if there is no user config file.

pkgs/applications/misc/deadd-notification-center/default.nix

@@ -43,9 +43,15 @@ in mkDerivation rec {
   # Test suite does nothing.
   doCheck = false;
 
-  # Add systemd user unit.
+  postPatch = ''
+    substituteInPlace src/NotificationCenter.hs \
+      --replace '/etc/xdg/deadd/deadd.css' "$out/etc/deadd.css"
+  '';
+
+  # Add systemd user unit and install default style.
   postInstall = ''
     mkdir -p $out/lib/systemd/user
+    install -Dm644 style.css $out/etc/deadd.css
     echo "${systemd-service}" > $out/lib/systemd/user/deadd-notification-center.service
   '';