Hi Ingrid,
you can try experimenting with this CSS:
body .grid-container {
max-width: 50vw;
}
.inside-header .grid-container,
.inside-navigation .grid-container {
max-width: 840px !important;
}
The first rule sets the main body container to 50% of the viewport width. So adjust that accordingly. The second rule should keep the header and logo as it is currently.
VW – Viewport Width is treated differently by different browsers when zoomed, so may require some experimenting.