Loads classic theme styles on classic themes in the editor.

This is needed for backwards compatibility for button blocks specifically.

function wp_add_editor_classic_theme_styles( $editor_settings ) {
	if ( WP_Theme_JSON_Resolver::theme_has_support() ) {
		return $editor_settings;
	}
	$suffix = wp_scripts_get_suffix();
	$classic_theme_styles = ABSPATH . WPINC . "/css/classic-themes$suffix.css";

	// This follows the pattern of get_block_editor_theme_styles,
	// but we can't use get_block_editor_theme_styles directly as it
	// only handles external files or theme files.
	$classic_theme_styles_settings = array(
		'css'            => file_get_contents( $classic_theme_styles ),
		'__unstableType' => 'core',
		'isGlobalStyles' => false,
	);

	// Add these settings to the start of the array so that themes can override them.
	array_unshift( $editor_settings['styles'], $classic_theme_styles_settings );

	return $editor_settings;
}