Background image/color struggles

My page

Hi guys,

How the hell does one create the background as seen in the photo below? I have the Curvy ‘SVG’ here

I’ve tried creating a div container that spans the top 20% of so but this just covers the SVG/PNG image.

[edit] Funnily enough I’ve looked for people who have completed this challenge and no one seems to have done the background. Whether that’s because it seems tricky I don’t know!

Can you use the svg as the background image, and add a dark gray background color?

I could if it were long enough but adding a background-color doesn’t reach the top.

I’ve added a gradient but it’s more work than seems needed.