Edit: background.php

<?php
/**
 * Background block support flag.
 *
 * @package WordPress
 * @since 6.4.0
 */

/**
 * Registers the style block attribute for block types that support it.
 *
 * @since 6.4.0
 * @access private
 *
 * @param WP_Block_Type $block_type Block Type.
 */
function wp_register_background_support( $block_type ) {
	// Setup attributes and styles within that if needed.
	if ( ! $block_type->attributes ) {
		$block_type->attributes = array();
	}

// Check for existing style attribute definition e.g. from block.json.
	if ( array_key_exists( 'style', $block_type->attributes ) ) {
		return;
	}

$has_background_support = block_has_support( $block_type, array( 'background' ), false );

if ( $has_background_support ) {
		$block_type->attributes['style'] = array(
			'type' => 'object',
		);
	}
}

/**
 * Renders the background styles to the block wrapper.
 * This block support uses the `render_block` hook to ensure that
 * it is also applied to non-server-rendered blocks.
 *
 * @since 6.4.0
 * @since 6.5.0 Added support for `backgroundPosition` and `backgroundRepeat` output.
 * @since 6.6.0 Removed requirement for `backgroundImage.source`. A file/url is the default.
 * @since 6.7.0 Added support for `backgroundAttachment` output.
 *
 * @access private
 *
 * @param  string $block_content Rendered block content.
 * @param  array  $block         Block object.
 * @return string Filtered block content.
 */
function wp_render_background_support( $block_content, $block ) {
	$block_type                   = WP_Block_Type_Registry::get_instance()->get_registered( $block['blockName'] );
	$block_attributes             = ( isset( $block['attrs'] ) && is_array( $block['attrs'] ) ) ? $block['attrs'] : array();
	$has_background_image_support = block_has_support( $block_type, array( 'background', 'backgroundImage' ), false );

if (
		! $has_background_image_support ||
		wp_should_skip_block_supports_serialization( $block_type, 'background', 'backgroundImage' ) ||
		! isset( $block_attributes['style']['background'] )
	) {
		return $block_content;
	}

$background_styles                         = array();
	$background_styles['backgroundImage']      = $block_attributes['style']['background']['backgroundImage'] ?? null;
	$background_styles['backgroundSize']       = $block_attributes['style']['background']['backgroundSize'] ?? null;
	$background_styles['backgroundPosition']   = $block_attributes['style']['background']['backgroundPosition'] ?? null;
	$background_styles['backgroundRepeat']     = $block_attributes['style']['background']['backgroundRepeat'] ?? null;
	$background_styles['backgroundAttachment'] = $block_attributes['style']['background']['backgroundAttachment'] ?? null;

if ( ! empty( $background_styles['backgroundImage'] ) ) {
		$background_styles['backgroundSize'] = $background_styles['backgroundSize'] ?? 'cover';

// If the background size is set to `contain` and no position is set, set the position to `center`.
		if ( 'contain' === $background_styles['backgroundSize'] && ! $background_styles['backgroundPosition'] ) {
			$background_styles['backgroundPosition'] = '50% 50%';
		}
	}

$styles = wp_style_engine_get_styles( array( 'background' => $background_styles ) );

if ( ! empty( $styles['css'] ) ) {
		// Inject background styles to the first element, presuming it's the wrapper, if it exists.
		$tags = new WP_HTML_Tag_Processor( $block_content );

if ( $tags->next_tag() ) {
			$existing_style = $tags->get_attribute( 'style' );
			if ( is_string( $existing_style ) && '' !== $existing_style ) {
				$separator     = str_ends_with( $existing_style, ';' ) ? '' : ';';
				$updated_style = "{$existing_style}{$separator}{$styles['css']}";
			} else {
				$updated_style = $styles['css'];
			}

$tags->set_attribute( 'style', $updated_style );
			$tags->add_class( 'has-background' );
		}

return $tags->get_updated_html();
	}

return $block_content;
}

// Register the block support.
WP_Block_Supports::get_instance()->register(
	'background',
	array(
		'register_attribute' => 'wp_register_background_support',
	)
);

add_filter( 'render_block', 'wp_render_background_support', 10, 2 );

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