This is a great question due to the fact that it correctly identifies discrepancies that exist between the scientific definition and usage of the word "theory" and how the general public defines and uses the word.
Scientifically speaking, a theory is a unifying explanation for a wide range of hypotheses and observations that have been supported by thorough testing. This means that a scientific theory is backed up and supported by large amounts of evidence. A scientific theory is not a whimsical guess about how or why something works the way that it does. This definition and usage runs counter to what the general population believes.
In my experience, the general population is quite quick to discredit an idea because "it is only a theory." Too many people assume a theory is an idea off the top of somebody's head that hasn't been tested and is not supported. That assumed definition is much closer to what a scientific hypothesis would be. A hypothesis is a possible explanation or answer to a question. Another way of thinking about a hypothesis is that it is an idea that you can test. A hypothesis that has been tested over and over again to confirm the same set of predictions can be used to form a theory.
While a theory does have supporting evidence, new evidence can always be brought in to modify and change the theory. Some theories can even be proven completely false, as was the case with geocentric theory, but the key is that any existing theory has supporting evidence found through scientific testing.