How to create a centered element on top of other elements?

Question

PyQt

Hey! How can I create a centered element in PyQt5, which, after clicking on the button, would appear in the center of the application and slightly darken all the elements around? This is something like registration elements on sites, when you click on the "Register" button and you have a window (sometimes in the center, sometimes only horizontal align center), which darkens the entire area and is, as it were, above the others (in css this is a parameter z-index).