I'm no stranger to patterns— in fact, I welcome as many as possible (even all at once) like with this outfit. On paper, mixing polka dots, stripes, and plaid sounds absurd. However, when done subtly, you can pull it off without calling too much attention to your outfit. Since the contrast of the polka dots on my sweater and plaid on my pocket square are low, I can go a little bolder with the contrast of the stripes on my shirt with ease.
All the aforementioned patterns can be seen in this single detail shot, but given their small doses and subtleties, the look doesn't feel too wild (even with a maroon jacket)