First, you don't have to put your form templates under acs-templating. Yes, it's not exactly elegant, but this works:
<formtemplate ... style="../../../package-key/www/template-file"></formtemplate>
Second, it's good to be able to have different themes on different subsites, but it should be used sparingly. One of my goals is to increase consistency across the system.
One distinction that we may want to make is that you can have different layouts, i.e., grid, list, table, whatnot; and then, for each layout, you can have several different graphic designs for each theme, but they all share the same layout, and they're all useful for the same purpose.