Linux commands to set solid color background.
If you’re familiar with Linux command line, press Ctrl+Alt+T on keyboard to open a terminal window.
When it opens, run the commands below one by one to disable current wallpapers for light and dark mode:
gsettings set org.gnome.desktop.background picture-uri ''
gsettings set org.gnome.desktop.background picture-uri-dark ''
In case you’ve set gradient color background before, reset the “color-shading-type” via command:
gsettings reset org.gnome.desktop.background color-shading-type
Finally, run the command below in set a solid color background (total dark for example):
gsettings set org.gnome.desktop.background primary-color '#000000'