Layout header swap

I am very new to WordPress, and I’m slowly but surely figuring things out, but there’s one thing I’m stuck on. If anyone can tell me how to switch headers from these two layouts, I would greatly appreciate it.

I want this header on this layout.

Specifically, I want the posts and sidebar to have a solid white background, no gaps or spaces between the two or the header, does that make sense?

Thanks in advance.