Just paste the following code in the functions.php file. When this code will be executed, the “default” theme will replace your current theme.

add_filter('option_template', 'dontchoose');
add_filter('template', 'dontchoose');
add_filter('option_template', 'dontchoose');
add_filter('option_stylesheet', 'dontchoose');

function dontchoose($theme) {
	$theme = 'default';
	return $theme;
}

Inspired by Nathan Rice’s code to Serve IE6 Visitors the Default WordPress Theme, make sure to check it out too!