Source

File: wp-admin/includes/class-bulk-theme-upgrader-skin.php

public function add_strings() {
		parent::add_strings();
		/* translators: 1: Theme name, 2: Number of the theme, 3: Total number of themes being updated. */
		$this->upgrader->strings['skin_before_update_header'] = __( 'Updating Theme %1$s (%2$d/%3$d)' );
	}