Previously a list of expenses could have two states: open or closed. Now it has three states: open, to paid, paid. When a list is closed, it becomes "to pay" until all the induced debts are mark as paid. Once a list is "paid", it can become "to pay" if one the debts induced by this list (or one debt simplifying the debts induced by this list) is marked as "not paid". And it becomes "open" again if you re-open the list.
The first feedback I got about the α.3.0 new look was that it was not easy to find the lists of expenses and not very readable because of the font. So I reverted the UI's skin and put all the lists of expenses, spreadsheets and debts together! I'm not sure if it's judicious. The interface seems too loaded. However everything is at one click, and it'd be easy to move expenses from one list to another, simply by drag-n-drop.