If I were to generate an address and it happens to collide, legally speaking, would a transfer constitute theft?
IANAL.

What you describe is the act of taking something that isn't yours in full knowledge that somebody else invested effort and money to accumulate it. 

Just as with your Swiss bank example: The rightful owner would be losing the funds that you misappropriated. Taking someone's property without their consent is theft.

I feel that this is independent of the way by which you gained access to the property, but rather derived from the intent of your action.

Perhaps more ambiguous would be scenarios where somebody receives a transaction to a key that you also control, or when you stumble on a key that has a Bitcoin balance that hasn't been moved for decades and could reasonably assumed to have been lost by the rightful owner.