Pre-styling post content to display in lightbox

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:

• Codex: Child Theme
• wp_enqueue_style