Any imaginative "theory" can be confected, with few constraints, from the knowledge, purposes or imagination of an interested party. But if it is to contribute to developing a practical theory, a theory-in-use, it needs be constrained by the capacities and situation of its user. Possible practice limits investigation. This is why many scientific disciplines demand some empirical "backup" for a hypothesis striving for the status of theory.
A theory-in-use is an amalgam of an object-theory, which is a rather idealized construction of a theorist, and user-theory, which can be defined by identifying the epistemological requirements of any practitioner employing the object theory. It is generally useless to theorize about what one cannot know. (Of course, trying to determine what one can or cannot know is often difficult and runs the danger of foreclosing on possible avenues to new knowledge.)
The primary use of theory by a practitioner is to rationalize or enhance the outcomes of her undertakings. Certain types of theory can be determined a priori to be practically useless: any object-theory which is non-conformable to user-theory has no primary use.
In practice, however, idiosyncratic "adjustments" are made by the user to the object-theory which make it appear to be usable. Terminology can be bandied about, but this results in little more than poetry -- or ideology -- even from the lips of a recognized scientist: no little temptation in our culture of promotion and hyperbole!
However, the usability of such "adjusted" theory does not indicate anything about the object-theory from which it derived.
To examine these issues further, see Using Theory