*googles*
3.6mg of Theobromine in a fresh cup of really strong tea.
LD50 in cats is 200mg/kg (no known lowest toxic dose).
On that basis, I suggest not panicking, but not encouraging a tea habit.
It's dogs that are usually the problem, as they're actually inclined to eat chocolate (dogs are inclined to eat pretty much anything, as far as I can see). Cats' sense of taste is wired for protein and fat, and they're uninterested in sweet things, so unlikely to do more than have one lick your chocolate and walk off in disgust. Not sure if there's an edge case with, say, chocolate ice cream, but that's not the sort of thing that you tend to leave lying around.