Hi, Shaz!
Absolutely. Have you watched the MSM Video? At the very end it shows exactly how the member functionality works between sites, and is better seen than explained, for sure.
But I’ll try to explain.
For the themes:
All that you need to do is head over to Admin -> Members & Groups -> Membership Profile Templates. Choose the site whose templates you wish to edit in the Sites drop-down. Now edit the templates as you wish. =)
For the default group, head to Admin -> Members & Groups -> Membership Preferences. Make sure you have the site you want to edit chosen (you can double check in the breadcrumb) and choose the default member group. Hit update. You can then choose a different site and choose a different default member group.
I hope that helps to clarify a bit. Let me know if you need more of an explanation. =)