Changing the sidebar width

OK I have read all the other posts but none of them seem to apply to my theme. My website is https://www.felixblog.co.nz. I need to modify the width of the sidebars to make them a little bit wider. I cannot find anywhere in the style.css or any of the other files under the editor where I can change the relevant settings.
Any help here is much appreciated and my son will be very grateful too as it’s his blog.

Cheers

Shaun