User gets a different theme that the one he chose with New Blog Templates

Hi,

I'm using New Blog Templates with Gravity Forms integration (GF User registration).
I'm using this must-use plugin @Jose developed:
https://premium.wpmudev.org/forums/topic/select-button-inside-templates-selector-doesnt-work-reloads-the-page/page/2#post-733877

Here is my problem:
When a user subscribes he doesn't get the template he chose.
- When the Default theme plugin is deactivated, he gets Twenty fourteen.
- When the Default theme plugin is activated and set on "Genesis [framework]", he always gets the same Genesis theme (which is not activated on the network).

By the way, I'm not absolutely sure that I don't need the Default theme plugin that's why I opened another thread about it:
https://premium.wpmudev.org/forums/topic/default-theme-and-new-blog-templates-together

Any idea?
Thanks!
Best Regards