Currently there is no plugin system.
I belive "trials" are simply a dummy install on a UBBCentral server.
Mods will have to be done manually and are against the support policy (though most support is done on this user to user forum or at UBBDev).
The style system uses a template system (smarty) and a css editor.
You can create custom portal modules or choose from the basic ones that ship.
