Overlay behind modal window in one line of CSS

Does this work if your modal has rounded corners?

Mm, not sure. But after googling it, border-radius and outline doesn't seem to play very well together. So maybe you have to stick to a square box.

I'm not able to test right now, but a box-shadow with a large spread shoild accomplish the same trick without border-radius issues. YMMV per browser, of course.