That dialog works fine in Lubuntu default theme because it has GTK3 support. Those engines you mention are not GTK3, they are GTK2, so the Update Manager (compiled in GTK3) doesn't work fine with them.
** Changed in: gtk2-engines (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Desktop Bugs, which is subscribed to gtk2-engines in Ubuntu. https://bugs.launchpad.net/bugs/1406807 Title: multiple engeines missing widgets in software updater on lxde To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gtk2-engines/+bug/1406807/+subscriptions -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs