Merge parent theme style variation with active child theme

functions.php

<?php

add_filter( 'theme_json_theme', 'get_parent_theme_style_variation_data' );
/**
 * Merges parent theme style variation data with active theme.json.
 *
 * @since 1.0.0
 *
 * @param mixed $theme_json WP_Theme_JSON_Data | WP_Theme_JSON_Data_Gutenberg.
 *
 * @return mixed
 */
function get_parent_theme_style_variation_data( $theme_json ) {
	$parent_style_variation = get_template_directory() . '/styles/dark-mode.json';
	$parent_style_json      = json_decode( file_get_contents( $parent_style_variation ), true );

	return $theme_json->update_with( $parent_style_json );
}