add_action( 'wp_enqueue_scripts', 'enqueue_styles' );
function enqueue_styles() {    
    wp_enqueue_style( 'bootstrap_css', 'https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css', false );    
    //wp_enqueue_script( 'popper', 'https://cdn.jsdelivr.net/npm/popper.js@1.16.1/dist/umd/popper.min.js', array('jquery'));
    wp_enqueue_script( 'bootstrap_js', 'https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js');  
}