pre > code {
    counter-reset: lines;
}

pre .codeblock-line {
    display: block;
    margin: 0;
}

pre .codeblock-line::before {
    content: counter(lines);
    counter-increment: lines;
    text-align: right;
    display: inline-block;
    width: 2rem;
    margin-right: 20px;
    color: dimgrey;
    border-right: 1px dotted dimgrey;
    padding-right: 3px;
}

/*pre .codeblock-line[data-highlight]::before {*/
/*    content: "+";*/
/*}*/


pre .codeblock-line[data-highlight] {
    background-color: darkgreen;
}