Customizing a header in Oxygen

I realize this question has been asked before, but the solutions they suggested didn’t help me.

The header on my website (diybudgetgirl.com) is supposed to be 800 x 125 px. Instead, WordPress automatically reduces it to the suggested size (which I think is 400 x 100). I’ve been trying to fix this in the Style.css file, but nothing changes.

I also cleared my browser cache, but it didn’t do anything.

This is what the code under “header” currently looks like:

#header {
	clear: both;
	width: 100%;
	float: left;
	padding: 40px 0 0 0;
}
#branding {
	float: left;
	width: 100%;
	overflow: hidden;
}
#site-title {
	font-family: 'Abel', 'Helvetica Neue', Helvetica, Arial, sans-serif;
	font-size: 3em;	/* 36 / 12 = 3 */
	line-height: 1em;
	margin: 0 0 3px 0;
	text-transform: uppercase;
	word-wrap: break-word;
}
#site-title a {
	color: #222;
	border-bottom: none;
}
#header img { max-width: 100%; }
#site-description {
	font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
	font-size: 0.9166666666666667em;	/* 11 / 12 = 0.9166666666666667 */
	font-weight: normal;
	line-height: 1.5em;
	margin-bottom: 20px;
	color: #bbb;
	float: left;
	width: 100%;
	clear: both;
}
.breadcrumbs {
	font-size: 0.9166666666666667em;
	color: #aaa;
	float: left;
	width: 100%;
	background: #f3f3f3;
	padding: .25em .6em .35em .9em;
	margin-bottom: 20px;
	border-left: 10px solid #e9e9e9;
}
.breadcrumbs a, .breadcrumbs a:visited { color: #777; }
.breadcrumbs a:hover { color: #000; }
#sidebar-header {
	overflow: hidden;
	float: right;
	width: 100%;	/* 468 / 940 = 0.4978723404255319 */
	height: 120px;
	margin-bottom: 20px;
}

I’m not very good with CSS, so I’ve done everything I can think of. There are so many files, I’m not even sure if I’m messing with the correct one. If someone can figure out what’s wrong, that would be appreciated.

Thanks!