css.css

a:not([href*="//"]) {
    /* CSS for internal links */
}

a[href*="//"]:not([href*="gitlab.it4i.cz"]):after {
    /*CSS for external links */
    background: transparent url("/img/external.png") no-repeat right 0px top 1px;
    background-size: 12px;
    padding: 1px 16px 1px 0px;
}