Remember, a scientific theory isn't the same as the colloquial definition of a theory (as a synonym for conjecture).
What most people think of when they use the word theory is what scientists call a hypothesis (a possible explanation which hasn't yet been tested, but is based on a knowledge of related phenomena) or an ad-hoc theory (an explanation based on general experience, available evidence, and guesswork without research into related documents), with a strong preference for the latter.
A theory is an explanation for why something happens in nature. Theories have been tested and reproduced and can be backed up with evidence; e.g. germ theory, General Relativity theory, and atomic theory. Additionally, the "explanation" part is important: the theory of evolution, for instance, is a set of explanation regarding how evolution works, evolution itself is a "phenomenon", i.e. something that has happened that has been directly observed.
And then there's a law which is a mathematical correlation observed repeatedly in many situations, usually reserved for the physical sciences; e.g. Newton's laws, Ohm's law, and the law of conservation of energy. Scientific laws are patterns in nature that theories seek to explain. Laws are not stronger than theories (i.e., theories that have been "proven" true). From Rationalwiki: A law of gravity will tell you that two objects will be attracted to each other and the magnitude of the force, contrastingly a theory of gravity will offer an explanation for the existence of the force.
Therefore, accusing something of being false merely because it's "just a theory" is fallacious reasoning, as the "theory" label doesn't make it any less valid or true. Additionally, theory, technically, plural, a singular explanation should be a "theorem": a scientist referring to "gravitational theory" is referring to the collective body of research on the subject of gravitation, not a specific law or explanation.
Also, theories are considered incomplete (but workable) by default as they are based only on observable data, and the ability to observe data is limited by instrumentation. Thus, theories change according to new data observed as the instruments of observation improve. While these changes are often pointed to as proof of the earlier theory being completely "wrong" (and thus, so is science), in reality the new data actually builds upon the old theory, forming a more complete but still incomplete theory. Simply put, being incomplete does not immediately prove any theory wrong. Gravity is a good example of this, with Newton's model still being correct to a very high degree at the speeds and masses observable by an unaided person, with quantum and relativity theory only adding more precision to size and speed scales only recently observable.
That said, however, the theory of gravity according to Newton indicates that gravity is actually a type of force which increases proportionally to the size of the object in question, while quantum and relativity theory implying that what we comprehend as the force of gravity is actually these masses causing the fabric of space to "bend" - to put it simply - around it. However, this is just a different way of looking at gravity. So obviously a new set of explanations for a particular phenomenon doesn't just add on to a previous one, but revises our understanding of it.
To combat some of this confusion of terminology, Richard Dawkins invent the word "theorum" to distinguish between the core knowledge of science (evolution, heliocentrism, and the Earth being round) from both other scientific theories and the layman's conception of "theory." "Theorum" as a word has failed to catch on among scientists.
Compare Windmill Political: Depending on the setting and context, this trope can be for those who believe in gravity, those who don't, or those who believe in something else that the author is poking fun at by comparing it to gravity.