Preheat broiler. Put bell peppers on rack of broiler pan about 5 inches from heat and broil, turning occasionally with tongs, until skins are blackened, about 15 minutes. Transfer to a bowl and let stand, covered, until cool enough to handle. Peel peppers over bowl (to catch any liquid) and discard stems and seeds. Cut peppers lengthwise into 1-inch-thick strips and add to liquid in bowl with vinegar, thyme, and capers. Add salt and pepper, to taste, and toss. Divide arugula among 4 plates. Top with peppers and drizzle with any remaining dressing. Serve at room temperature or chilled. Note: Dressed peppers can be made 3 days ahead and chilled, covered.