Commit c72b8b21 fixed the styling of the active window's button,
but missed that the `active` property uses the style information
as well.
Adjust it to use the correct actor when checking for the style class.
Closes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/529
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/342>