During the Campaign setup process, you can enable your Senders to choose the theme of the gift emails they send.
In step 3 of setting up the Campaign, you will see an option to Allow sender to choose theme.
Campaigns used to be called "Touches". Read about the change here.
Sendoso Choice Campaign set up:
When you check this box, your Senders will be able to select a theme from your Customization Center.
Inventoried Campaign set up:
To learn more about how to choose a theme when the send is initiated, check out this article: How do I change the theme of my gift email?