Replace dynamic size of overlay with a 100% width and height

I had a bug with overlay : the overlay was not resized on window resize when a modal was open, it seems that the event was unassigned somewhere.

This fix don't fix the bug, but prevent it : we don't need to resize the overlay because we set to him a 100% width / height. Since the overlay is attached to the body with a fixed position, CSS is enough.

Plus, I moved the top / left = 0 assignement from the javascript to the CSS (better for a user who want to customize these values).

Regards