Combine cucumbers, onions, green bell peppers, red bell peppers, and salt in a large container. Cover with ice cubes. Let stand for 3 hours. Drain off brine and squeeze out excess moisture.
Mix sugar, vinegar, mustard seeds, and celery seeds in a saucepan. Bring to a boil; cook for 3 minutes. Pour over cucumber mixture. Let cool, about 30 minutes.
Divide cucumber mixture among jars. Cover with lids and refrigerate.
