Resizing content area

I’m using the tarski theme and I’m trying to resize the main frame I believe it’s called the content area (sorry i’m really new to wordpress). I don’t really understand code at all but i’m good with computers so there were a few things i figured out on my own like which area of the code i need to change, i just don’t know how to do it right. if anyone could help it would be greatly appreciated

here is a link to my site and to the css

WH Land Design

Style.css