Background color not set on website but ok in Post Template

Using WP 6.4.2 with Spectra One.
Everything is great, and working as wanted, but I have a problem with the background color of my posts (don’t know if it’s my newbie knowledge of WP or a bug).
In the Single Post Template I set the background color of the container between the header and footer.

But when I load the post on the website, only the text of the post is of that color. The surrounding rest is white and I don’t know, still after many tries to get the background color everywhere on the post page.

Any clue?

Thanks