Nice themes for your Linux desktop
Window themes
Google Chrome Metacity theme for MATE and Gnome Shell.
This is a nice Google Chrome inspired theme for MATE and Gnome Shell. This is a very good theme.
https://www.opendesktop.org/s/Mate/p/1006898/.
Nice colourful Windows 8.1 theme for Metacity and Gnome Shell.
https://www.opendesktop.org/s/Mate/p/1006881.
Icon themes
Papirus.
https://www.opendesktop.org/s/Mate/p/1166289.
A very colourful and bright icon theme for MATE. This would suit and colourful desktop environment.
Windows 10 icon pack
https://www.gnome-look.org/p/1215834/. This is great if you want to emulate the Windows 10 look on Ubuntu.
Windows Vista icon theme. Works very well with the MATE desktop.
https://www.securitronlinux.com/linux/files/Vista.tar.gz.
Professional GTK theme for MATE and Gnome.
https://www.gnome-look.org/s/Gnome/p/1334194.
Desktop wallpapers
Make the Cinnamon desktop look like Windows 10.
https://securitronlinux.com/debian-testing/make-the-cinnamon-desktop-look-like-windows-10/.