Automatically activating theme after WordPress installation

To activate the theme that you want to be activated automatically after WordPress (version 3.0 and more) installation, copy that theme to the /wp-content/themes/ directory, then open the file default-constants.php from the directory /wp-includes/ and in the function wp_templating_constants change the value of WP_DEFAULT_THEME to the name of the theme directory.

Example:
define( 'WP_DEFAULT_THEME', 'mortgage' );