Some very nice themes for your Linux desktop in 2021
Gnome Shell themes
MetalX Gnome Shell theme for Linux.
https://www.deviantart.com/art/Metal-X-theme-v-2-1-for-Gnome-3-16-577064989.
Android Flat Gnome Shell theme. A nice flat grey theme for your Gnome desktop.
https://www.deviantart.com/art/Android-Flat-3-18-565622252.
Ambiance Gnome.
https://www.deviantart.com/art/Ambiance-Gnome-3-14-1-8-GNOME-Shell-theme-476996106.
An Ubuntu inspired theme for any Gnome Shell desktop. Make your Gnome Shell desktop look like Ubuntu Unity.
Elegance-Elemantary-Faience-Axiom Gnome Shell 3.6.
https://www.deviantart.com/art/Elegance-Elemantary-Faience-Axiom-Gnome-Shell-3-6-335120156.
Gorgeous Gnome Shell theme.
Wallpapers
Golden Gate bridge quad monitor wallpaper.
An abandoned building in Pripyat, dual monitor wallpaper.
Ford GT-40 on a race track.
Icon themes
Faience icon theme set.
http://tiheum.deviantart.com/art/Faience-icon-theme-255099649.
Papirus Icon Theme
http://www.omgubuntu.co.uk/2016/08/best-linux-icon-theme-papirus-supports-more-apps.
The Papirus icon theme, which is derived from Sam Hewitt’s Paper theme, already supports hundreds of well known apps, including all the usual faves like Firefox, Thunderbird, Corebird, GIMP and Geary.