Websearch says that LD50 (the overdose that will kill half the animals in the study) for caffeine is around 70 mg per 1 lb body weight (150 mg / kg body weight). As an example, instant coffee contains over 60 mg caffeine per teaspoon. So, the lethal dose of caffeine for a 15 lb (6.8 kg) dog would be 17 teaspoons of instant coffee. Stronger coffees such as Turkish coffee, percolated coffees, and energy drinks would take much less to become toxic for a dog. LD50 in a cat is 200 mg/kg ... So cats are less sensitive per body weight, but do typically weigh less.
The theobromine LD50 is about 1000 mg/kg in humans. But for cats it’s 200 mg/kg and for dogs it’s 300 mg/kg. So cats are more sensitive to tea and chocolate than dogs.
However, cats are much less likely to scarf down significant quantities of either than dogs are. They don't react to sweetness, among other reasons.
So you don't want to actively feed your cat these chemicals. But a few drops, or a few chocolate flakes, are unlikely to do any harm.