cant add custom background without removing theme

[Yanko Pam]

i want to edit this theme"

https://chrome.google.com/webstore/detail/when-the-night-falls-3-ra/aapgnjnokhcaonaedihiihmbeiaddgdo

and add a custom background to it. chrome removed my image some time ago along with the ability to change the background without altering all the other aspects of the theme. help