Well, the width of the sidebar is determined here:
The main content width is set by this rule:
margin: 0 30.4% 0 0;
The navigation menu width is determined here:
box-shadow: 5px -5px 5px -5px rgba(0, 0, 0, 0.5);
margin: 0 auto;
Setting the width of the #secondary-wrapper to 20% and the margin of the #content-wrapper to 20.4% worked for me.
You then need to set the width of the navigation menu. Setting it to 79.6% worked for me.
You'll want to play around with the figures to get a set of widths that you're happy with.
If you don't have full control over the theme's code, you should seriously consider making your changes in a Child Theme. It's the only recommended way of modifying a theme.