Create your own child theme containing a functions.php
and style.css
so if your third party theme gets updated, you do not loose any work you put into it.
In the functions.php
enqueue your stylesheet.
function enqueue_my_style() {
wp_enqueue_style( 'xtheme-custom', get_stylesheet_uri() );
}
add_action( 'wp_enqueue_scripts', 'enqueue_my_style' );
In your style.css
write down your CSS mark-up of style adaptions.
Sources: