theme: tone down unfocused title labels

- mimic libadwaita in toning down unfocused titles. Cargo culting the
  filter instead of using normal opacity property.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/4468

gtk/theme/Default/_common.scss

@@ -1500,6 +1500,8 @@ headerbar {
     background-image: none;
 
     transition: $backdrop_transition;
+
+    .title { filter: opacity(.5); }
   }
 
   .title {