Place chicken thighs in the bottom of a five-quart slow cooker. In a small bowl, combine sugar, soy sauce, vinegar, garlic, ginger, and pepper. Pour over chicken, and cook (covered) on low for 4-5 hours or until chicken is tender. Remove chicken and place on a serving platter, and keep warm. Transfer cooking juices to a small saucepan, skimming fat as needed, and bring to a boil. In a small bowl, stir together cornstarch and cold water until smooth. Add to cooking juices, and return to a boil, cooking and stirring for 1-2 minutes or until thickened. Serve sauce with chicken over rice or vegetables, if desired.