Background box ?

Hi guys,

I`m just starting to build a page with this beautiful theme and I was wondering if is possible to achieve this kind of thing:

https://imageshack.com/i/idq4Dgcyj

To have a background image / or color and a “box” behind the page which can have different color and opacity.

Thanks in advice for any suggestions !