Register LESS stylesheet using the WP LESS plugin, download it here: http://wordpress.org/plugins/wp-less/. Register stylesheet(s) explained here: https://github.com/oncletom/wp-less/wiki/Common-Usage#Registering%20a%20LESS%20stylesheet.

functions.php

// Register LESS stylesheet using the WP LESS plugin, see https://gist.github.com/Willem-Siebe/e7800869c3517d85614c.

function wsis_enqueue_styles() {
    wp_enqueue_style('style-less', get_stylesheet_directory_uri().'/less/style.less');
}

add_action('wp_enqueue_scripts', 'wsis_enqueue_styles');