Repeating background image

Hi,
I’m a little embarrassed to ask this, I know little to nothing about html and web design.:hang:

I want a webpage’s content to be within the grey strip shown in this image. The grey strip has gradient edges on the left/right.

The way I went about doing it is to save a horizontal strip of the grey area as an image (800 width x 10 height), and tile it vertical and center. The documents background color is black.

Is this the correct way to do it? It seemed to do what I wanted it to.