Think of theory as a model. It's a model that's made to fit evidence, facts, observations, logic, math, rules, knowledge, etc.
For instance, when we drop thing, they fall to the ground. We can model how, when, how fast, etc things fall, and then call it a model of "things falling to the ground" or more popular "gravity." In other words, a theory (or model) is not something that is a complete wild guess without any supporting facts or data. No, a theory/model is something that is modeled out of evidence. Sometimes (or most times) it's not perfect. It's a model. Just like a toy model of a real train engine won't be 100% perfect, but it's darn close! Just because it's a model doesn't mean it's completely wrong. Just because it's not 100% doesn't mean it's 0% correct.
Put it this way, your English isn't the greatest. My English isn't great either, but does that mean that we are always wrong? If I spell one word wrong, does it mean that all my words are spelled wrong? Of course not! The same with a model. There are parts and pieces that are wrong in a model, but it doesn't mean the whole model is wrong.
Take the law of motion for instance. That if you're on a moving flatbed truck and throw a ball the speed of the ball is the force you throw it with and the speed of the truck. You add them together if you throw in the same direction as the truck is moving. But... that's totally wrong!!! Because of special relativity and the limitation of speed of light, that formula is oversimplified. But it works in the given context of less than 1/10th of speed of light. So is the model completely wrong or is it mostly right and only wrong in specifics?