Where am I supposed to put --uc-newTab-wallpaper value?

[seaque]

user_pref("userContent.newTab.background_image", true); // Need wallpaper image --uc-newTab-wallpaper: url("../icons/background_image.png");

I tried this in userChrome.css but wallpaper isn't changing.

:root { --uc-newTab-wallpaper:url("linktomyimage"); }

[black7375]

You can add :root { --uc-newTab-wallpaper:url("linktomyimage"); } to userConent.css.
It's not clear what this means. I'll update it.