Hit Enter and the control is updated. You won't see the changes unless you open the style dialog but if you save, the updated styles are saved. Now, click another control in the designer, up arrow in Dev Tools and hit Enter to update it too.
I haven't worked out how to refresh the control after the change but at least you can save the changes. 🙂 Of course this is highly likely to destroy your view in a firey blaze of useless XML, so have a Cancel Checkout ready.