use grey shadow instead of transparent black

Author: tobyhodges

assets/css/lesson.scss

@@ -302,7 +302,7 @@ span.fold-unfold {
 
 
 img.image-with-shadow {
-  box-shadow: 0 6px 24px 0 rgba(0, 0, 0, 0.25);
+  box-shadow: 0 6px 24px 0 #888;
 }
 
 //----------------------------------------