Sidebar and main page width.

Look here: https://www.themesandco.com/snippet/changing-the-global-column-layout-of-the-customizr-theme/

Hope it helps.

Thanks for the link. but I do not understand what it says exactly. Where should I check it? style.css? function.php?
I am a pretty beginner. Sorry.

Check?
No you have to use that snippet in your child-theme functions.php, changing the value of those spans for “b” (which stays for “both sidebars” => your case).