This code will remove the borders around WordPress gallery images and change the layout model to CSS flexbox.

Very Simple FlexBox override for WordPress Galleries.css

.gallery {
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -ms-flex-wrap: wrap;
        flex-wrap: wrap;
    -webkit-box-pack: justify;
        -ms-flex-pack: justify;
            justify-content: space-between;
}

.gallery br {
	display:none;
}

.gallery-item {
    -ms-flex-preferred-size: 16%;
        flex-basis: 16%;
    margin: 0 !important;
}

@media only screen and (max-width: 760px) {
  body .gallery-item {
      -ms-flex-preferred-size: 32%;
          flex-basis: 32%;
  }
}

.gallery-item img {
    border: none !important;
}