Avoid falling back to Adwaita with no way to set dark variant.

The check is not perfect and will fail if libhandy and GTK prefixes differ.
Nothing we can do about that though.

Also it will stop picking up changes until restart if theme has been
invalid even once. This is unfixable too, since we can't track changes
on GdkScreen settings directly, and as soon as we set gtk-theme-name is set
it can't be used to track changes anymore.

Still better than nothing.