Hello wooly2, All themes for Firefox version 57 and above are lightweight (background) themes. They only cover the toolbars. Complete themes are no longer supported. See : https://support.mozilla.org/en-US/kb/use-themes-change-look-of-firefox But maybe you'd like to try what's described here : https://www.techuntold.com/how-change-firefox-background-theme-wallpaper/ There are some add-ons, but I'm not sure if they'll do what you want : https://addons.mozilla.org/en-US/firefox/addon/site-color-changer/ And : https://addons.mozilla.org/en-US/firefox/addon/togglewebsitecolors/

See also this extension: *https://addons.mozilla.org/firefox/addon/new-tab-tools/