Classic style for the WordPress Gallery block

functions.php

<?php

// CSS adjustments may be necessary for the caption, depending on your theme.
add_action( 'init', function() {
	register_block_style( 'core/gallery', [
		'name' => 'classic',
		'label' => __( 'Classic', 'theme-textdomain' ),
		'inline_style' => '.wp-block-gallery.is-style-classic.has-nested-images {
			align-items: flex-start;
		}
		.wp-block-gallery.is-style-classic figure.wp-block-image figcaption {
			position: initial;
			margin: 0;
			padding: 0.5rem;
			color: inherit;
			background: unset;
		}'
	] );
} );