Adwaita: backport nightly styling

- nightly apps get a headerbar styling

gtk/theme/Adwaita/_common.scss

@@ -1770,6 +1770,17 @@ headerbar { // headerbar border rounding
   @extend %titlebar;
 }
 
+// Development versions of apps to use a differently styled headerbar
+
+window.devel {
+  headerbar:last-child {
+    background: transparent -gtk-icontheme("system-run-symbolic") 90% 0/256px 256px no-repeat,
+                linear-gradient(to left, mix($selected_bg_color, $bg_color, 10%) 8%, $bg_color 25%);
+    color: transparentize($fg_color,.9);
+
+    label { color: $fg_color; }
+  }
+}
 
 /************
  * Pathbars *