Hey Jane,
first of all, thank you for the purchase!
The line you’re after is around 200th (cannot tell for sure in case you’ve made changes to the file) in the theme’s style.css: it should say “#page-background { … }”. You need to change the “z-index” property for that element to “-2000” for it to hide behind the full-page slider.
Let me know if you have troubles applying the fix!
Best,
Andrew / Satori Studio