Adwaita: Set a background for magnifiers

The magnifier in entries is magnifying the GtkText
widget, which does not draw any background itself.
Therefore, we give the magnifier a background, to
make things look as expected.

Fixes: #3318

gtk/theme/Adwaita/_common.scss

@@ -1845,6 +1845,9 @@ popover.background {
   }
 }
 
+magnifier {
+  background-color: $base_color;
+}
 
 /*************
  * Notebooks *