Theme applied to marketpress stores

Hello,
I can not activate a desired theme for a created store through marketpress.
I had installed Buddypress too and followed the advices in https://premium.wpmudev.org/forums/topic/how-to-build-your-own-etsy-like-site-using-marketpress.
I noticed that the loaded theme for a created theme is always twentyThirteen but as a fake path, the base theme is not loaded (that is another problem)
So my main question is how activate a theme on a created store ?
I network enabled several themes without any results on facebiz.fr which theme (buddypress default) is different of a sub store ex : facebiz.fr/def8. So I do not understand why it is the twentythirteen theme which seems to automatically be linked a all new stores.
Thanks for your help