add firefox-gnome-theme derivation

derivations/firefox-gnome-theme/default.nix

@@ -0,0 +1,25 @@
+# Taken from https://github.com/tadfisher/flake/blob/main/pkgs/firefox-gnome-theme/default.nix
+
+{ lib, stdenv, src }:
+
+stdenv.mkDerivation {
+  pname = "firefox-gnome-theme";
+  version = "unstable";
+
+  inherit src;
+
+  dontConfigure = true;
+  dontBuild = true;
+  doCheck = false;
+
+  installPhase = ''
+    mkdir -p $out/share/firefox-gnome-theme
+    cp -r theme/* $out/share/firefox-gnome-theme
+  '';
+
+  meta = with lib; {
+    description = "A GNOME theme for Firefox";
+    homepage = "https://github.com/rafaelmardojai/firefox-gnome-theme";
+    license = licenses.unlicense;
+  };
+}