Add a Class Conditionally for WordPress Page Templates

functions.php

function prefix_conditional_body_class($classes) {
	if ( get_page_template_slug(get_the_ID()) === 'mytemplate.php' ) {
		$classes[] = 'bg-dark';
	}
	return $classes;
}

add_filter( 'body_class', 'prefix_conditional_body_class' );