gtk-3.0: Fix Nautilus places sidebar

This fixes Nautilus's places sidebar which had an opaque grey area
where the overshoot/undershoot transparent areas were meant to be.

gtk-3.0/gtk-widgets.css

@@ -3664,6 +3664,12 @@ GtkProgressBar.osd.progressbar {
     background-color: @osd_bg;
 }
 
+/* notably used in Nautilus */
+.sidebar GtkPlacesSidebar {
+    /* fixes opaque grey area for the overshoot and undershoot areas */
+    background: rgba(0, 0, 0, 0);
+}
+
 /******************************
  * destructive action buttons *
  ******************************/