Hey Pablo,
Have you tried removing the max article width? This should allow the articles to extend to the edge of the page.
If that doesn’t work, check whether there are max widths under Page > layout as these may impact this too.
Final thing to mention: watch out for the Article Content Max Width. This is different to the Article Max Width and that can be a bit confusing.
I haven’t had a close look at your example but my guess would be that there’s no Article Max Width or Page Max Width but there may still be an Article Content Max Width set. This would limit the article content (the text and graphic) running right up to the edges of the article like the article background does.