front-columns margin-right

This isn’t working (parabola\includes\theme-frontpage.php):

#front-columns > div.ppcolumn:nth-child(<?php echo $parabola_nrcolumns; ?>n) { margin-right: 0; }

The old line of code works:

#front-columns > div#column<?php echo $parabola_nrcolumns; ?> { margin-right: 0; }

For now I fixed it with this (using 3 columns):

#front-columns > div#column3 {
    margin-right: 0;