How to set a content border

Are you wanting the shadow around the entire container, or just around individual boxes (content, widgets, header etc..).

Let me know :)

Just shadow around the entire container, thanks

This article should help: http://docs.generatepress.com/article/adding-a-container-wrapper/

... something is not working.
I followed all that instructions, and this is the result:
http://www.sr22insurancealaska.org/

The border-shadow is not around the container.

Thanks

My mistake, just updated the doc.

You want your opening element to be:

<div class="full-container grid-container grid-parent">

Then in your case, you can remove this CSS:

.site-header {
    margin-top: 20px;
}

And add this:

.full-container {
    margin-top: 20px;
}

Thanks, it has worked very well!

Last question please.

WHen in the future I will update the theme and/or the premium plugin, will I lose all this modifies?

I mean, exactly all these modifies:

1) CSS code added in APPEARANCE > CUSTOMIZE > Additional CSS
2) code added in APPEARANCE > GP Hooks

Thanks

No, you won't lose those changes.

Updates remove and upload new core files, so as long as you don't touch those, you're safe :)