Use the plugin WordPress MU Domain Mapping. Read the installation instructions very carefully. There is also an ebook WordPress Multisite 101 where you can read more (like migration from single site installation etc.).
You can activate themes per single site as usual. Or set the constant WP_DEFAULT_THEME
in your wp-config.php
depending on the host name:
switch ( $_SERVER['HTTP_HOST'] )
{
case 'example.com':
define( 'WP_DEFAULT_THEME', 'blue-child' );
break;
case 'example.net':
define( 'WP_DEFAULT_THEME', 'red-child' );
break;
default:
define( 'WP_DEFAULT_THEME', 'parent-theme-slug' );
}