Expanding editing area

Currently I'm using the Fulton template and everything is great except I don't like how narrow the editing area is. If I want to stretch a text box as wide as possible, it only goes up to border of the editable area which fills up about a third of the page whereas other templates such as Sonora have a much wider editing area that go almost up to the edge of the website.

I think it could be fixed with CSS but I don't really have any experience there.

Please help!

Fulton doesn't have that option, but Sonora does. However, when I checked the Sonora template it still didn't solve the problem that I have.