Add missing drop shadow to theme previews (#13888)

Author: willdurand

src/amo/pages/Addon/styles.scss

@@ -58,6 +58,7 @@
     padding: 24px;
 
     .ThemeImage {
+      box-shadow: 0 4px 4px 0 rgba(0, 0, 0, 12%);
       margin: 0 auto;
     }