Place the cabbage and salt into a large colander, toss to combine, and set in the sink to drain for 1 hour. Add the peppers, toss to combine, and drain for an additional hour. Rinse thoroughly under cool water for at least 1 minute, transfer to a salad spinner to dry. Transfer the mixture to a 1-quart canning jar with a lid.
In a small saucepan, bring the vinegar, sugar, mustard seed, and celery seed to a boil. Pour over the cabbage and the peppers. Store in the refrigerator for 3 days before serving. Keep in the refrigerator for up to 2 months.