This should do, within the terminal:
cd ~/.config/gtk-3.0/
touch gtk.css
ls -la
EDIT: @dav69, you’ll need instructions from this important post: Terminal to clipboard - #8 by Pink_Unicorn, although Ctrl+c and Ctrl+v will work on Librem 5 keyboard very well.
One way is to rename your background picture to mybackground.jpg and put/paste it in here: /home/purism/Pictures/ or just “Change the path to point to your chosen image”.