Spacing between flexbox items

You can try setting the same margin for all the boxes, and then revert this on the container:

So replace this:

.flex > * { margin: 0 10px; }    
.flex > :first-child { margin-left: 0; }
.flex > :last-child { margin-right: 0; }

.flex.vertical > :first-child { margin-top: 0; }
.flex.vertical > :last-child { margin-bottom: 0; }

With this:

.flex.vertical { margin: -20px 0 0 -20px; }
.flex > * { margin: 0 0 0 20px; }
.flex.vertical > * { margin: 20px 0 0 0; }

The CSS spec has recently been updated to apply gap properties to flexbox elements in addition to CSS grid elements. This feature is supported on the latest versions of all major browsers. With the gap property, you can get what you want with just gap: 10px (or whatever size you want).