[Theme: Mantle] change content and sidebar width

This is a child theme of GeneratePress.
I actually use in my blog http://www.digitalking.it

I would like to decrease the size of the content and increase the size of right sidebar to fit better.
I know that there is the Spacing paid addon but in another support topic i saw that is possible via a filter.

https://wordpress.org/support/topic/decrease-the-width-of-sidebars-2?replies=9#post-7062738

I used the first part of code of Tom reply putting in the functions.php of the child theme but when i browse the site it returns error 500.

Any help really appreciated thanks!