Help learning how to go about making this background

Hello Im a noob coder and needing some help here.

my site is site

The following link is an image of what I want the background for my site to look like.

img
With all the content living in that red area and the grey being what expands with the browser size(dunno lingo).

Essentially I am not sure how to do this and what would be the smartest way to go about doing it. Can I get some help? Thanks in advance.

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