It’s possible, but it’s currently designed to use the standard 1110px width that Discourse content exists within.
You could add CSS like this to make the content on pages with the sidebar wider
@media screen and (min-width: 1110px) {
.navigation-topics,
.categories-list,
.tags-page,
body[class*="category-"]:not(.archetype-regular):not(.archetype-banner) {
.wrap {
max-width: 1400px;
}
}
You could even go really crazy and set something like max-width: unset; and have a browser-wide topic list
