How does a theory differ from a theory

A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not "guesses" but reliable accounts of the real world.

Both of those terms are widely used in higher education, particularly in the sciences. Their technical definitions, as well as their common underlying meanings, are precisely identical. So you can use either one without any fear of being corrected or misunderstood.