I bought a used Croma from 2017 (pre facelift).

It has roof rails fitted, that I do not need. I would like to take them off!

Can I do this myself? Or is it complicated?

The roof rails can be removed. They bolt on from the "top side" so there is no read to drop the interior head lining.

However you will the be left with needing replacement "filler strips" to run in the roof line channel. These will not be that cheap and will need painting.

If I were you I would just leave then asis.