How to add 10px between images using CSS?

If we use box-sizing: border-box; then you can just apply borders to specific elements without breaking the layout.

you could make the entire gallery 10px wider than the screen and have a 5px padding (that is for two images in a row).

Alternatively you might have a padding around the images a have the left images receive a relative position of -5px horizontally and the right images a relative position of +5px horizontally. The tricky part would be to get the padding/offset right so that the separation in the center will also be 10px.

.sr-gallery li {
    margin: 0;
    padding: 0;
    float: left;
    width: 33.33%;
    overflow: hidden;
    text-align: center;
    border-bottom: 10px solid white;
    box-sizing: border-box;
}

.gallery-col2 li {
    width: 50%;
    border-right: 5px solid white;
}

.gallery-col2 li + li {
    border-left: 5px solid white;
    border-right: none;
}