rofi: add theme option

This is helpful when defining a `*.rasi` theme for rofi using
`pkgs.writeText` rather than messing up the impure `~/.config`
directory.

pkgs/applications/misc/rofi/wrapper.nix

@@ -0,0 +1,17 @@
+{ stdenv, rofi-unwrapped, makeWrapper, theme ? null, lib }:
+
+stdenv.mkDerivation {
+  name = "rofi-${rofi-unwrapped.version}";
+  buildInputs = [ makeWrapper ];
+  preferLocalBuild = true;
+  passthru = { unwrapped = rofi-unwrapped; };
+  buildCommand = ''
+    mkdir -p $out/bin
+    ln -s ${rofi-unwrapped}/bin/rofi $out/bin/rofi
+    ${lib.optionalString (theme != null) ''wrapProgram $out/bin/rofi --add-flags "-theme ${theme}"''}
+  '';
+
+  meta = rofi-unwrapped.meta // {
+    priority = (rofi-unwrapped.meta.priority or 0) - 1;
+  };
+}