Hi there,
you can:
1. Make the Blog page full width using a Layout Element:
https://docs.generatepress.com/article/layout-element-overview/
2. Use a Content Template to build your post design.
https://docs.generatepress.com/article/block-element-content-template/
The first part of that design would include a Container Block, set its inner width to what you require. And give it a CSS class of custom-post-container
in Advanced > Additional CSS class(es)
Then build your post card inside that.
Once thats done we can sue some CSS to change background colors for every odd container eg.
.gb-container.custom-post-container:nth-child(odd) {
background-color: #ccc !important;
}