customizing marketpress and still keep updates

I love MarketPress. Thank you. I am modifying it to fit my needs (a gift registry) and need to add a few functions.

My site is a multisite installation. The goal is to make it work like Etsy without using the backend and act as individual gift registries.

One of the functions I'm adding is a shortcode that will display the products from the Main site and a button that will allow them to add them to their inventory.

My question is, can this function go in my theme functions.php file or will it need to go in the template-functions.php?

Second is that for style purposes, I would like to change the background color of each product as they are displayed. So I did the following inside the mp_list_products function:

//add last css class for styling grids
			if ($count == $last)
			  $class = array('mp_product', 'last-product');
			  $class = 'mp_product';

			  if( $odd = $count%2 )
					// $odd == 1; the remainder of 25/2
					$class .= ' mp_odd';
					$class .= ' mp_even';

Is MarketPress designed so we can hook to it?

I'm a relately new WP developer and just learning the ropes.

Please advise.

Thank you,