Heat the oil in a large nonstick skillet over medium-high heat. Add onion and garlic; cook 4 minutes or until onion is tender. Add the chicken; cook 4 minutes. Add curry powder and marjoram; cook 1 minute. Add tomato, broth, and pepper; reduce heat, and simmer 15 minutes. Remove from heat.
Combine yogurt and flour; stir with a whisk, and add to chicken mixture. Cook 1 minute or until slightly thick. Serve mixture over couscous; top with raisins, if desired.