So we can style it differently than :hover.
We already have a active state for the menu items which includes
more than hover. For example, when the keyboard focus moves to a item
or we select programatically a item.
For this reason we need a style class named active for the meaning we
give to it in menu items, and a pseudo class active with the meaning
CSS has.
https://bugzilla.gnome.org/show_bug.cgi?id=744680
Since widnows decorations with round corners has a special shape,
which when scaled we can't match with a simple border-radius.
So to prevent to show a gap between the caption border and the
caption itself, apply a shadow.
