You need to register and/or enqueue your stylesheet and hook it to wp_enqueue_scripts with a priority high enough that it loads after the Woothemes default.
function load_my_style_wpse_87804() {
wp_register_style( 'mystyle', get_template_directory_uri() .'/path/to/mystyle.css');
wp_enqueue_style( 'mystyle' );
}
add_action('wp_enqueue_scripts','load_my_style_wpse_87804',100);
You may need an even higher priority (third parameter), but eventually you should get one high enough.
This is, of course, assuming that Woothemes and the slider enqueues properly.