How can I make civibuild use "Seven" as the default Drupal theme instead of "Bartik"?
Two options: 1) edit your own buildkit script  or 2) prepare a PR for buildkit and convonce Tim seven is a lot more useful than Bartik; I'll second that!

A third could be a config option; 3) PR a change to buildit where the theme you like become a config option when running amp config