Change page width of child theme

Hi, I would like to change the width of all my webpages which is using this theme. I have created a child theme. I followed steps to create the page.php file in my child theme and have changed the “site-main large-8” to “site-main large-12” in the page.php file. However, when I reload my site, nothing changes. I have also tried to change the .row {max-width: 75.125em; } to .row {max-width: 120.125em; } in the foundation.css file. Unfortunately, nothing changes as well. I have disabled my cache plugins. Could anyone guide me on how to change to wider page width correctly? Your help is much appreciated.