For a page you write, you can choose the type of layout you want given that your theme offers that. However, bbPress runs on a page created on the fly, three is no page called “forums” unless you create one and then use shortcodes to bring up the forum.
Every theme has a CSS file which can be edited within WordPress. Go to Appearance/Editor and your theme CSS should load. I edited my theme’s CSS and added the code I gave in my previous post.