Trouble making sidebar wider

I am using the fashionistas theme and I can’t seem to make my sidebar wider. I’ve been messing with it for awhile now and I can’t make sense of it. Every time I change anything it drops to the bottom left corner.

What I’m trying to do it change it so that the sidebar is wider than the widgets, and this is the code:

/* LAYOUT
----------------------------------------------- */
.container {
	width: 980px;
	}
.content-area {
	width: 100%;
	float: left;
	}
.site-content {
	margin: 0 340px 0 0px;
	border-right: 3px solid #5F526B;
	padding-right: 30px;
	}
.site-sidebar {
	width: 300px;
	float: left;
	margin: 0 0 0 -300px;
	background-size: cover;
	background-position: fixed;
	background-image: url("https://www.happieducks.com/wp-content/uploads/2016/02/sidebar-background.png");
	}