Here is a truly elegant theme for our Desktop Gnome o Xfce, which is available in 2 variants, one with light colors and of course, the other with dark colors.
The theme is compatible with gtk3, but, for them we must have Unico installed, the gtk-engine which comes by default in Ubuntu. For gtk 2 we just have to have installed pixbuf y Murrine:
$ sudo apt-get install gtk2-engines-murrine gtk2-engines-pixbuf
We can download it from this link.