Hello,

You can easy add cutom css code in your WP Admin -> Appearance -> Customize -> Additional CSS: http://prntscr.com/gbgkc9

/*--------------------------------------------------------------
16.1 Galleries
--------------------------------------------------------------*/
.gallery-item {
    display: inline-block;
    text-align: left;
    vertical-align: top;
    margin: 0 0 1.5em;
    padding: 0 1em 0 0;
    width: 50%;
}
.gallery-columns-1 .gallery-item {
    width: 100%;
}
.gallery-columns-2 .gallery-item {
    max-width: 50%;
}
.gallery-item a,
.gallery-item a:hover,
.gallery-item a:focus {
    -webkit-box-shadow: none;
    box-shadow: none;
    background: none;
    display: inline-block;
    max-width: 100%;
}
.gallery-item a img {
    display: block;
    -webkit-transition: -webkit-filter 0.2s ease-in;
    transition: -webkit-filter 0.2s ease-in;
    transition: filter 0.2s ease-in;
    transition: filter 0.2s ease-in, -webkit-filter 0.2s ease-in;
    -webkit-backface-visibility: hidden;
    backface-visibility: hidden;
}
.gallery-item a:hover img,
.gallery-item a:focus img {
    -webkit-filter: opacity(60%);
    filter: opacity(60%);
}
.gallery-caption {
    display: block;
    text-align: left;
    padding: 0 10px 0 0;
    margin-bottom: 0;
}
@media screen and (min-width: 30em) {
    /* Gallery Columns */
    .gallery-item {
        max-width: 25%;
    }
    .gallery-columns-1 .gallery-item {
        max-width: 100%;
    }
    .gallery-columns-2 .gallery-item {
        max-width: 50%;
    }
    .gallery-columns-3 .gallery-item {
        max-width: 33.33%;
    }
    .gallery-columns-4 .gallery-item {
        max-width: 25%;
    }
}
@media screen and (min-width: 48em) {
    /* Gallery Columns */
    .gallery-columns-5 .gallery-item {
        max-width: 20%;
    }
    .gallery-columns-6 .gallery-item {
        max-width: 16.66%;
    }
    .gallery-columns-7 .gallery-item {
        max-width: 14.28%;
    }
    .gallery-columns-8 .gallery-item {
        max-width: 12.5%;
    }
    .gallery-columns-9 .gallery-item {
        max-width: 11.11%;
    }
}

Thank you