halo-theme-fantastic/source/lib/prism/css/prism-line-numbers.css

pre.line-numbers {
    position: relative;
    padding-left: 3.8em;
    counter-reset: linenumber;
}

pre.line-numbers > code {
    position: relative;
    white-space: inherit;
}

.line-numbers .line-numbers-rows {
    position: absolute;
    pointer-events: none;
    top: -1.8rem;
    padding-top: 1.6rem;
    font-size: 100%;
    /*background-color: #EBEDED;*/
    left: -3.8em;
    width: 3em; /* works for line-numbers below 1000 lines */
    letter-spacing: -1px;
    -webkit-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
    bottom: -2rem;

}

.line-numbers-rows > span {
    pointer-events: none;
    display: block;
    counter-increment: linenumber;
}