If you increase the width of only one of two side by side divs in CSS, you would most presumably also need to decease the other.
In order to widen the sidebar, you’ll need to narrow the site-content area, even if it’s just by a tiny bit. The sum of all the components cannot be wider than the total width. You have to account for borders and padding and margins content-area, sidebar and all that good stuff.
So if you reduce the site-content to 65% or thereabouts you can increase the width of the sidebar.
Hope that makes sense to you.
Rebecka, thanks!!!!!!! I ended up changing everything to percentages instead of fixed px values, and it worked!!!!!