Where to insert post_class function in WordPress single.php file?

Hello,

I’d like to be able to change background colour of posts based on their category, and based on my research I understand that post_class function is what I need to use:

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

I’m editing single.php file of my Ippo child theme. I’ve been unsuccessful so far in understanding where exactly the div + function should go.

My theme is Ippo (child), and the single.php file looks like this:

<?php
/**
 * The template for displaying all single posts.
 *
 * @link https://developer.wordpress.org/themes/basics/template-hierarchy/#single-post
 *
 * @package Ippo
 */

get_header(); ?>
	
	<div class="container container-medium">

		<?php while ( have_posts() ) : the_post(); ?>

			<div class="hero clear">
		    	<!-- Featured media -->
		    	<?php ippo_featured_media(); ?>

		    	<div class="entry-header">
		    		<!-- Entry header -->
		    		<?php ippo_entry_header( 'single' ); ?>
	    		</div>
			</div>

			<div class="row">

				<div id="primary" class="content-area <?php ippo_content_classes(); ?>">
					<main id="main" class="site-main" role="main">

						<div class="container container-small">

							<?php get_template_part( 'templates/template-parts/content', get_post_format() ); ?>

						</div><!-- .container.container-small -->

						<?php

							// Author box
							ippo_author_box();

							// Post navigation
							ippo_post_navigation();

							// Related posts
							ippo_related_posts();

							// If comments are open or we have at least one comment, load up the comment template.
							comments_template();

						?>

					</main><!-- #main -->
				</div><!-- #primary -->

				<?php get_sidebar(); ?>

			</div><!-- .row -->

		<?php endwhile; // End of the loop. ?>

	</div><!-- .container.container-medium -->

<?php

get_footer();

I’ve already changed the style.CSS to include:

.category-SPRING {background: #000000; border: 1px solid #FFFFFF;}

And on the WP site have a post under category SPRING.

I’d appreciate any suggestions! Thanks.