Chop the leaves of 2 bunches rainbow chard; slice the stems. Brown 3 slices chopped bacon in olive oil in a large pot. Add the chard stems, 1 chopped red onion, 1/3 cup golden raisins and a pinch of salt. Cook over medium-high heat until softened, 4 to 5 minutes. Stir in 2 tablespoons cider vinegar and 1 teaspoon sugar. Add the chard leaves in batches and cook until wilted, 2 to 3 minutes. Season with salt and pepper.