Can you apply portfolio theory to forex market?

Think of forex (or cryptocurrencies) trading as a creating a portfolio (a "basket" of currencies) and maintaining its good structure by planned rebalancing. It also allows to reduce net risk (diversification! Markowitz like it!). E.g. now I have five positions - pairs with absolutely different currency symbols. Some trades are lossy, some are profitable. All you need in this situation is to manage active positions properly and control risk.