I use Lubuntu 12.04 and a gtk2 theme, Elegant Arch.
With such a theme, gtk2 applications like Thunar will be themed, while others like Nautilus, would be not (see this question ant the images there).
Because I wanted to theme gtk3 applications like Nautilus, Audacious etc., I have copied the "gtk-3" folder from the main folder of a gtk3 theme into the main folder of my Elegant arch theme, which is in home/username/themes. Using the gtk3 folder from this full flat theme I got a pretty similar look in gtk2 and gtk 3 apps.
Lubuntu Desktop Center, Ubuntu Desktop Center, Network Options and other such apps are now themed (before they were not). Even Update Manager is.
But why is Synaptic Manager not themed?