FullStack Dev to fix and update website with pages extracted from WordpressTheme.Firebase Backend.

I have created a web page that is extracted from a wordpress theme. I used the simply static plugin to generate static web pages. These static web pages- enable me to edit the html and hook up javascript as well as php to the website so that the pages fetch data from Firebase Realtime Database. Upon selecting a car, one is redirected to a second page that shows details of a car. Among other details is the photo of the vehicle which is set as the background of a certain <div> I have been unable to change the background using php,jquery and javascript. This is the primary problem I would like assistance with. Further edits and updates can be discussed based on the completion of the above task. You should be knowledgeable with Firebase Realtime Database Web Development, Wordpress JavaScript and any other web technologies e.g JS frameworks.