functions.php

<?php

add_action( 'after_setup_theme', 'my_add_filter' );

function my_add_filter() {
    add_filter( 'pless_vars', 'my_pless_vars' );
}
function my_pless_vars( $vars ){
    $vars['plBaseFont'] = '"ff-tisa-web-pro",georgia,serif;';
    return $vars;
}