Removing twentyfourteen sidebar

Hi all,

I’m trying to remove the left sidebar. Unlike many other I want to remove it only on certain pages.
So I’ve created a new page template where I’ve removed the get_sidebar() call. Now I’m left with a black rectangle on the left side of my blog.

How do I get rid of it?
I know it comes from “site:before”.

What I don’t understand is how to add an if statement to style.css (as far as I know it’s impossible). At this point i’m stuck. All the guides out there talk about removing it completely…

(No site address as I’m testing on my local dev server)

Thanks