Support Nictitate: http://kopatheme.com/forums/topic/override-extra-css-properties/

functions.php

<?php

add_action( 'wp_enqueue_scripts', 'kopa_nictitate_child_theme_customize_extra_style' );

function kopa_nictitate_child_theme_customize_extra_style() {
	wp_enqueue_style('kopa-child-theme-extra-style', get_stylesheet_directory_uri() . '/css/extra.css', array('kopa-extra-style'), null );
}