Widen all my pages and decrease the sidebar

Hi I´m using Catch flame theme. The free version. I want to widen all my pages and decrease the sidebar. That’s all I want to do. Do I really need to create Child pages? I have installed the plugin “Child pages” of Jan Henckens but I don´t know how to use it. What I see in this forum you always suggest “Custom CSS box”. I don´t have a “Custom CSS box” regards Lena