There are two ways now to change the default theme. If you are already in CRXDE, then just look in the themeRef property under /conf/my-templates/settings/wcm/templates/blank/initial/jcr:content/guideContainer. Change this property to point to the theme you want, click Save All and you are done.
Or in the Coral Template Editor, you can edit the Initial Content of the template using the drop-down on the top right. Select the Adaptive Form Container and click the wrench icon. You should now see the default settings for the new template Adaptive Form container. Select the Theme you want to display in the new form dialog by default.
Test out your new template by creating a new Adaptive Form and select your new template - your default theme should show as whatever you changed it to.