Not sure if you already override the CERN theme, but you will have to in order to make the page wider. In order to override the cern theme, you need to install the CERN Override theme.
After installing the CERN Override theme, you need to override the page template by creating a twig file in /templates/ named page.html.twig. So in total your structure should be templates/page.html.twig. Here is a useful documentation about how template overrides work.
Specifically about what you want to achieve, you should remove the left and right sidebars and make the central area to hold 12 columns. CERN theme is based on bootstrap and you can read more about the bootstrap grid and columns in this documentation.
Hope that helps. Let me know if you have more question or if it doesnt work.