In my mind design and theory are inseparable. Design is the accumulation of many design decisions. Theory explains what influenced those decisions.

Design needs theory to be intentional. It can of course be accidental (”seems to work, I guess”) or intuitive (”i know in my guts this is right but cant explain it”).

While both can end up with functional systems, if you cant vocalize the design journey the system is not very maintainable in the industrial sense (hence - theory is the vocalization of the design and the forces that influenced it).