Adjusting the Content / Sidebar Widths

We’ve carefully constructed the grid, so it should be very easy to customize. By default, the content area is 64% wide, that translates to 640px wide. The sidebar area is 30% wide, which translates to 300px wide.

So, to make the content area wider, you simply subtract from the sidebar, whatever you add to the content area or vice versa depending on what you’re trying to do. So, if for example, you want a wider content area and we made it 70% wide, you would need to reduce the sidebar to 24% wide.

#content{width:70%}
#sidebar{width:24%}

https://startupwp.com/topic/customizing-your-theme/