Sure. That’s
section.gh-content.gh-canvas {
grid-template-columns: [full-start] minmax(var(--gap),auto) [wide-start] minmax(auto,240px) [main-start] min(var(--content-width,720px),calc(100% - var(--gap)*2)) [main-end] minmax(auto,240px) [wide-end] minmax(var(--gap),auto) [full-end];
}
It’s the content-width you’d need to change, so replace min(var(--content-width,720px),calc(100% - var(--gap)*2))
with something like min(var(--content-width,900px),calc(100% - var(--gap)*2))
to make it a little wider.
Don’t forget to surround with <style></style>
tags.
If it doesn’t seem to be applying, you may need to add an !important, but I think the selector above is specific enough that it should work without it.