This works
The theme settings are under settings DESIGN > WEBSITE STYLES. The one you want is Site Max Width. It used to be 1280px. (You can always reset at the bottom if you mess it up).
If you want to change the theme settings back, then just go into the page you want to be 100% and go in "page settings", "advanced" and then "header code injection" and just paste this in:
.collection-type-page .site-page {
max-width: 100%;
}Alternatively, you could also navigate to DESIGN > CUSTOM CSS and put it there, then it'd show up on every page, but I can't think of a reason why it's better than the theme setting.
Note: editing CSS may not always work as the theme could update and cause it to stop working. Using theme settings with services like this is always better.