Theme Per Group

Provides a per group theme functionality for the Group modules. In many ways, it is equivalent to OG Theme module for Organic Groups.

Dependency : Group

This module will show all the themes which are enabled.

HOW TO USE :

step 1: Download the module.
step 2: Create all the Group types before enabling the module.
step 3: Now enable the module and a field will appear on all group types.
step 4: Select a theme from a list.

Another Way to add the field in new group type if you already enabled the module.

If you enabled the module and then you create a new Group type and you want to add theme field to this group.

Go to manage fields of that group type in which you want to add the field > manage fields > add existing field > select_theme_group (machine name of the field) and do not change the machine name.
Make the field required, and leave the default setting for the field.
Save the field and you are done.