Main Site Responsiveness Issues

-I was checking responsiveness of my website on different screen resolutions (dimensions).
-I used the website https://quirktools.com/screenfly/ for testing and also resizing manually the browser windows.(Results are the same)

-On some screen resolutions it looks like this. It lacks margins left and right

-Even the default generatepress theme lacks the margins on those resolutions

-But Here is a website build with generatepress that has the margins at those resolutions and its responsive for all screens.

-To achieve that i introduced this css

.container {
        margin-left: 15px;
        margin-right: 15px;

-It solved the problem but back a full browser window, the site container is squashed to the left and not centered again. see main site

-How can i remedy that???