Hi there,
if you want to apply a % width it will require CSS that targets the page-id-*
body class. For the page you linked to it would be this:
body.page-id-1507 .grid-container {
max-width: 80%;
}
You may want it inside a media query so it doesn’t effect smaller device screens like so:
@media (min-width: 1200px) {
body.page-id-1507 .grid-container {
max-width: 80%;
}
}
If you want to use PX instead of % you can use the Layout Element to change the Content Width:
https://docs.generatepress.com/article/layout-element-overview/