Start off here $ sudo snap install bing-wall
BingWall does not look different: once installed it lets you download Bing’s featured photo and set it as the desktop background on your Linux desktop.
If you love a fancy and cool wallpaper, this is a must have.
Once installed, launch BingWall to browse through the various Bing images of the day and set them as your desktop wallpaper.
Enjoy 😊
Ubuntu uses Gnome as de (desktop environment). You can your gtk-theme to a dark theme. But you are talking about wallpaper in this case. You can also set your wallpaper within gnome setting daemon.