Hi - It's pretty much like any other plastic covered fixture. It uses tabs and plasticity to keep itself in place. You just need to overcome those factors to remove it.
Pinch the long sides together, essentially bowing the cover so that the side tabs can be pulled out of the frame. Then just lowering the non tabbed end to remove it from the frame.
I've removed mine and took a couple of photo's for reference.
Operationally our shower light only comes on when we use the switch outside the shower door. The light over the bathroom sink comes on by a separate switch located on the wall above the toilet.
All the best!
Tom - S/OS #025