We know theory of relativity and refraction too. theory of relativity states that speed of light is constant but according to refraction, the speed of light changed when it change its medium. then which one is true if both then answer fast

Theory of relativity shows that the speed of light doesn't depend
on the motion of the observer.  All observers measure the same
speed of light no matter how they're moving.

It does NOT say that the speed of light is the same in any medium.
If the light is moving through a block of jello, then all observers in jello
will measure the same speed of light, no matter how they're moving
and whatever that speed is.