Fallback/background image

Hello fellow wordpressers,

I am using the Alma theme, and in my homepage I show a video as the background. Everything so far works perfect.

I have assigned a fallback image for mobile, and even though the image appears firstly, when the site finishes loading it is replaced by a black background.

Where can I find CSS or PHP to hardcode the url for the fallback image or background on mobile devices?

Thanks!