Are there rules for either customized or completely custom-built themes?
There are three important considerations for any store planning to customize an IndieCommerce off-the-shelf theme, a purchased theme or a custom-designed and built theme.
First, the final theme must be compatible with Drupal 7.
Second, the final theme must not be dependent upon any Drupal module unsupported by IndieCommerce.
The IndieCommerce platform installation of Drupal 7 includes core and contributed Drupal 7 modules that we have tested. We will again review, on a case-by-case basis, additional modules that might benefit all users, but this does not alter the requirement that custom themes be free of dependencies upon unapproved modules.
You can find a list of currently supported modules here.
Third, in order to avoid unnecessary expense, confusion with developers or web designers and a customized site that does not function as part of IndieCommerce, notify the IndieCommerce staff of your theme plans in advance by writing firstname.lastname@example.org.