function custom_scripts() {
$theme = get_field('global_theme', 'option');
$stylesheet = ($theme == 'alt' ? '/styles/style.css' : '/styles/style-alt.css');
wp_enqueue_style( 'custom-style', get_template_directory_uri() . $theme );
}
add_action( 'wp_enqueue_scripts', 'custom_scripts' );