Hi,
It appears that you made a theme layer by mistake.

Create a top-level layer, choose layout in the dropdown, and keep the core version at 2.

I forgot you should also make a theme layer (Use Fluid Measure (######) where the set of characters are, you'll actually find your layout layer's number. It will be below the official layouts)

Paste in Forest Green's theme code and then you'll find the background is one solid color. Don't forget about the #header css in the css box.