Why is CaO a lewis base

We know that $\ce{CaO}$ is a base. But when I wanted to know which theory proves this fact, I got to know that Lewis theory explains this through a reaction with $\ce{SO\_3}$. But the definition of Lewis base are those substances which can donate lone pair electrons. We all know $\ce{CaO}$ is an ionic compound. So how can it possibly have lone pair electrons? Moreover, there is no central atom in $\ce{CaO}$. So whose lone pair electrons will we be counting if there were one?


