Hi there,
try this instead:
1. Customizer > Layout > Container – and set the Padding to 20px all round.
2. Use this for your shadow CSS instead:
.masonry-post .inside-article {
-webkit-box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.75);
-moz-box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.1);
box-shadow: 0px 2px 5px 2px rgba(0,0,0,0.1);
}
Note – Masonry use JS so changes within the Customizer may not reflect the front end until the browser is resized / refreshed.