This is a good theme for your modern Ubuntu or Linux Mint desktop, a version of the Human theme for GTK 3.0: http://gnome-look.org/content/show.php?content=164227. This is a perfect theme to make your Ubuntu desktop look like the old Ubuntu 8.10 releases.
Here is another lovely GTK 3.0 theme, Dorian: http://gnome-look.org/content/show.php/Dorian+Theme?content=157200.
A collection of old Ubuntu wallpapers: http://ubuntu.ecchi.ca/wallpapers/. These will complete the look of a classic Linux desktop on a modern distribution.
Here is another page, this is a archive of all of the default wallpapers for Ubuntu releases from 4.10 to 14.10: https://andrew.kvalhe.im/ubuntu-wallpaper-archive/.
