Currently firefox doesn't allow me to get to the point of creating a theme background as well (but could I be the one who hasn't succeeded yet); On the web though there are many backgrounds with cubes that you can add in firefox (definitely from version 130 onwards, maybe even earlier). Here I'll explain: On the top right of the firefox homepage there's a little gear icon. Click there and you can change the background to the homepage.
Read more: https://support.mozilla.org/en-US/kb/customize-your-wallpapers