Responsive layout not working when using custom site width

Hello all, I just wanted to make the layout wider. From 1000px to 1150px. In the style.css file I saw the responsable setting:

.site {
	width: 1150px;
}

The responsive layout is only working when I edit the original file. But: It is not working when I am using 1. a child theme 2. the customize css style option in theme editor Is this a "bug" or my fault? Kind regards André