Changing space between images in gallery

I am trying to increase the amount of space between each image in my gallery but I can’t find the code necessary to do so. I keep changing this part of the CSS to no avail:

.gallery-item {
float: left;
overflow: hidden;
position: relative;
margin: 0 4px 4px 0;
margin: 0 0.4rem 0.4rem 0;
text-align: center;
}

Here is a sample gallery so you have a better idea of what I am trying to change:
https://chianunwa.com/943-2/

The page I need help with: [log in to see the link]