A scientific theory can always be disproved if someone comes along with better evidence which shows another theory is true. However, most other theories are small improvements on existing ones. Evolution and natural selection (the theory that animals' populations change over time because their environment encourages specific different features for individuals that happen with mutation) was an improvement on the existing "theory" that animals were built to be fit for their environments. The earth rotating around the sun was a theory improving on the idea that the sun and the earth move around so that it looks like the sun moves around the earth. All of these happened because there was evidence, so while theories aren't always absolutely true, many modern theories are typically well-tested and if they aren't, they are usually refered to as hypotheses or models (although models can sometimes also be theories, like the Standard Model)
