Custompress with Marketpress, FrameMarket and GridMarket

I want to add some custom fields to product posts in a multisite network where Marketpress and GridMarket are not active on the main blog. Which theme do I need to edit to add the embed codes in the WordPress loop, GridMarket, or FrameMarket?

Also, since it is a multisite network, will that affect every instance of the edited theme across the entire network?