Follow-up from "system-modal-dialog: Tweak system-modal-dialog CSS"

The following discussion from !760 (merged) should be addressed:

• @alexander.mikhaylenko started a discussion: (+2 comments)

  What I mean with box-shadow is sth like:

  box-shadow: 0 0 0 1px alpha(black, .75);

@tobias.bernard should we do the same as Gtk's dialog here?