Is it possible to create a translucent rectangle?

[Vispixad]

A new problem has appeared for me, I am creating my own project where I would like to dim the interface to display a notification, but is there such a possibility? Namely, creating a translucent rectangle which will overlap other elements but at the same time they will be visible under it but with a darker or lighter color (depending on if the rectangle is dark, light or another color, for example red).