Wallpapers
Here is a nice selection of lovely 3440×1440 wallpapers for the Linux desktop.
3440×1440+ Wallpapers dump (ultrawide)
These would look very good on a Linux Mint MATE or KDE desktop.
Themes
CDE/Motif theme for Xfce4: https://www.gnome-look.org/p/1231025.
Mac OSX Mojave theme for GTK 4: https://www.gnome-look.org/p/1275087
ChromeOS Gnome Shell theme for Linux: https://www.gnome-look.org/p/1333760
Icons
Buuf Plasma: https://www.gnome-look.org/p/1305826.
Buuf for many desktops: https://www.gnome-look.org/p/1012233.
Mac OSX Catalina icons: https://www.gnome-look.org/p/1309810.
GDM Themes
High Ubuntuterra: https://www.gnome-look.org/p/1207015. A Mac OSX-styled GDM theme for your Linux computer.