For sauce, stir together yogurt, lemon peel, lemon juice, cilantro, paprika, ginger, garlic, cumin, turmeric, cardamom, and cayenne pepper in a shallow dish. Reserve ¼ cup of the mixture, cover, and refrigerate until serving time. Add chicken thighs to the remaining mixture and turn to coat. Cover and marinate the chicken in the refrigerator for 12 to 24 hours.
For a charcoal grill, place the chicken on the grill rack directly over medium coals. Grill, uncovered, for 20 to 25 minutes or until no longer pink (180°F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place the chicken on grill rack over the heat. Cover and grill as directed.)
Meanwhile, cook couscous according to package directions in a medium saucepan, omitting any salt or oil. Stir in cilantro, pine nuts, currants, lemon peel, garlic, and salt.
Serve the chicken with the reserved yogurt sauce, the couscous, and, if desired, lemon wedges.