Grav main site theme with wide display

@sam2020, To give you a tad more explicit solution...

• First, if not already, create an inherited theme. If you don't, you will risk loosing all changes when you refresh Antimatter, or install an update.
  Let's presume you have created an inherited theme called 'mytheme'.
• To remove the sidebar from the main blog page:
  • Copy file 'user/themes/antimatter/templates/blog.html.twig' into folder '/user/themes/mytheme/templates'
  • Open file 'user/themes/mytheme/templates/blog.html.twig'
  • Remove or comment out lines 30-32
• To remove the sidebar from a blog item:
  • Copy file '/user/theme/antimatter/templates/item.html.twig' into folder '/user/themes/mytheme/templates/'.
  • Open file '/user/theme/mytheme/templates/item.html.twig'
  • Remove or comment out lines 12-14
• To keep the content at a max width of ~1100px + ~150px padding:
  • Create file 'user/themes/mytheme/css/custom.css'
  • Add the following to 'custom.css':
    CSSCopy

    @media (min-width: 1300px) {
    .padding-horiz,
    .fullwidth #header,
    .fullwidth #breadcrumbs,
    .fullwidth .blog-header,
    .fullwidth .blog-content-item,
    .fullwidth .content-wrapper,
    .fullwidth ul.pagination,
    .fullwidth #body > .modular-row,
    #body,
    #header,
    #footer {
    padding-left: calc((100vw - 1300px) / 2 + 7rem);
    padding-right: calc((100vw - 1300px) / 2 + 7rem);
    }
    }
    

    The above css will maximise the width of the content to ~1000px plus ~150px (7rem) of space at each site. Below 1300px it will follow the default layout of Antimatter.

Hope the above creates what you are looking for...