gtk3: Adjust libhandy popover scrollbar background

Use the $_popover_bg as a scrollbar background color, to avoid the
odd separation between the scrollbar and the popover boxpointer.

Author: jnsh

common/gtk-3.0/3.24/sass/_libhandy.scss

@@ -21,6 +21,8 @@ popover.combo {
   overshoot.bottom { @extend %linked_vertical_bottom; }
 
   scrollbar.vertical {
+    background-color: $_popover_bg;
+
     &:dir(ltr) { @extend %linked_right; }
     &:dir(rtl) { @extend %linked_left; }
   }