Орхан Гасанлы

<ul class="cards">
<li class="card">Запись 1</li>
<li class="card">Запись 2</li>
<li class="card">Запись 3</li>
<li class="card">Запись 4</li>
<li class="card">Запись 5</li>
</ul>

ul.cards li:nth-child(3n) {
background:red;
}

ul.cards li:nth-child(2n) {
background:green;
}

function register_styles_scripts() {
	
	//стили
	wp_register_style('test-bootstrapcss', get_template_directory_uri() .
		'/assets/bootstrap/css/bootstrap.min.css');
	wp_enqueue_style('test-bootstrapcss');

	wp_register_style('style', get_template_directory_uri() .
		'/style.css');
	wp_enqueue_style('style');
	
	//скрипты
	wp_enqueue_script('test-popper', '//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.6/umd/popper.min.js');

	wp_register_script('jquery.min', get_template_directory_uri() .
		'/assets/bootstrap/js/bootstrap.min.js');
	wp_enqueue_script('test-bootstrapjs');
}
add_action('wp_enqueue_scripts', 'register_styles_scripts');

background-image: url('/wp-content/uploads/image.jpg');
backgrount-repeat:no-repeat;

<?php $my_custom_image_url = get_field('my_custom_image_url'); ?>
<style>
.my_custom_class {
background-image: url('<?php echo $my_custom_image_url ?>');
}
</style>

<?php $my_custom_image_url = get_field('my_custom_image_url'); ?>
<div class="my_custom_class" style="background-image: url('<?php echo $my_custom_image_url ?>');">
<!-- Your content here -->
</div>