Hello Warwick

The portfolio thumbnails are actually arranged by the Packery library:

https://packery.metafizzy.co/options.html

You can control the margins between items with CSS:

.item {

    display: block;
    position: relative;
    box-sizing: border-box;
    z-index: 0;
    width: 30%;
    height: 38vw;
    float: left;
    margin: 80px 10%;
}

These are the default values. I bolded the properties you may want to change.

Hello Warwick

You should be able t do so

https://packery.metafizzy.co/options.html#element-sizing

the portfolio container is initialised in function Portfolio() from /js/scripts.js

$container.packery({
itemSelector: '.item',
gutter:0,
transitionDuration: "0.5s"
});