Having a background image in native mobile

Hi,   I have downloaded this project https://video.mendix.com/watch/me2c9Chfbwd9EvQMPkVysM given in this webinar. I was trying out things as I am not familiar with native mobile styling. I would like to know how did they do for their default home page.   I have tried going into: themes→ styles → native → js→ app → custom.js, copied their code from “export: backgroundImageContainer” as well as “overBackgroundImage” and pasted into a new dummy project: themes → styles→ native → app → custom.js. I just want to have a feel of how this works first before actually implementing myself.   I also changed the class name for the containers respectively. However, it did not go that well. May I know if I am doing the things correctly please?
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