I took this approach because I tried to reuse the WindowDraggingUtils code already there. In the case of generic toolbars, it shouldn't be any more difficult than another system metric plus a dab of extra CSS. I don't know of any themes so far that have completely unified toolbars or whether it does anything beyond GtkToolbar::window-dragging (such as ID's or ancestry checks).
I took this approach because I tried to reuse the WindowDraggingUtils code already there. In the case of generic toolbars, it shouldn't be any more difficult than another system metric plus a dab of extra CSS. I don't know of any themes so far that have completely unified toolbars or whether it does anything beyond GtkToolbar: :window- dragging (such as ID's or ancestry checks).