you are picking the common use of the word rather than the original usage especially as used in regard to scientific theories.
In science you can not get any more definite a statement than when it is called a theory because it only becomes a theory after it has been shown to be true with evidence and studies that are replicated by others in that field of study.
Without any evidence an idea is a hypothesis and some hypothesis should be considered just a story because they are so outside reality

https://www.dictionary.com/browse/scientific-theory

scientific theory
Word Origin
noun
a coherent group of propositions formulated to explain a group of facts or phenomena in the natural world and repeatedly confirmed through experiment or observation:
the scientific theory of evolution.

And in the case of evolution there is no doubt it is true as it has been proven over and over from many different areas of scientific study.
All evidence just keeps on proving the undeniable evidence that evolution is how all animals developed, indeed all life forms.