I personally prefer the OpenSolaris look for Gnome because it`s more clearly then Ubuntu`s Humanlooks.
It`s possible to install this theme under almost any system running Gnome.
The easiest way is installing the proper package from http://gnome-look.org/content/show.php/Nimbus+(+Debian+and+Ubuntu+packages+)?content=70212
This site contains the packs for Ubuntu 8.04, 8.10 and Debian (both x86 and x86_64).
Another way is compiling and installing the latest version. This is how it goes!
Install the proper requirements and the source;
apt-get -y install build-essential libgtk2.0-dev intltool icon-naming-utils fakeroot devscripts debhelper build-dep gtk2-engines-clearlooks
wget http://dlc.sun.com/osol/jds/downloads/extras/nimbus/nimbus-0.1.2.tar.bz2
tar -xf nimbus-0.1.2.tar.bz2
Next step; compiling the source
./configure --prefix=/usr/
chmod +x debian/rules
Install the binary;
make clean
make
sudo make install
Select the theme in Gnome.
Thats it!