Help with Fixed Background and Mobile / Desktop Banner Images on Dawn Theme

[PourStudios]

Is anyone good with basic CSS on Shopify? I need help with my store I am working on. I want the main banner image to be stay fixed with the background (parallax) but I have to have 2 different banners, one for mobile and one for desktop. I am using the Dawn theme on Shopify and here is what I want, the codes I'm using right now and what I'm running into. Before I added for the main image to be fixed with the background, the only issue I had besides it not staying as I scrolled was that the desktop banner was out of focus.

Can someone help me figure out how to keep the main banner on desktop in focus and keep it fixed in the background at full width?

Can someone also help me with keeping just the main banner on desktop fixed to the background so it doesn't scroll with the rest of the page?