Source

File: wp-admin/includes/class-wp-ms-themes-list-table.php

public function _order_callback( $theme_a, $theme_b ) {
		global $orderby, $order;

		$a = $theme_a[ $orderby ];
		$b = $theme_b[ $orderby ];

		if ( $a == $b ) {
			return 0;
		}

		if ( 'DESC' === $order ) {
			return ( $a < $b ) ? 1 : -1;
		} else {
			return ( $a < $b ) ? -1 : 1;
		}
	}