Change sidebar width?

I'm using the Simple Catch theme (free version). I was hoping to change the width of the sidebar to make it narrower and the main content wider (I currently have a right sidebar). I was successful in changing the width of the content itself by editing the css stylesheet, but not the overall space of the sidebar - the main content is the same width as it was before and the block for the sidebar is the same width as before, it just has more empty space. Is there a way to correct this?