wp-conditional-css.php

<?php

function my_css_function() {
	if ( is_single() && get_post_meta( get_the_ID(), 'field_key', true ) != '' ) {
		wp_enqueue_style( 'style-name', 'folder/stylesheet.css' );
	}
}

add_action( 'wp_enqueue_scripts', 'my_css_function' );