Fillmore considered that adjuncts can also be assigned theta roles, but the problem, as Dowty points out, is where to stop.
Also, it's unclear how we should decide which theta roles are ‘complement’ theta roles and which are ‘adjunct’ roles.
We would assign the theta role ‘agent’ to Steven and ‘patient’ to the book, since Steven is performing the action and the book is the recipient of that action.