BuddyPress.org

If I’m understanding you correctly, you want member blogs to use the same theme that you use for your home theme. If this is correct, here’s how you can accomplish that.

In wp-content >> themes, rename the default folder. Something like old-default works well so you remember what it is.

Then, rename the folder of the theme that you want to be set as default.

i.e. rename buddypress-home to default