Ah, I get it. The answer is usually "create a child theme!"
http://codex.wordpress.org/Child_Themes
In your wp-content/themes directory, create an empty directory called coraline-child. In that directory create a file named style.css and copy in these lines below into that file.
/*
Theme Name: Coraline Child Theme
Author: Self-Help WordPress User
Template: coraline
*/
@import url("../coraline/style.css");
/* Start child theme CSS here */
#header {
position: fixed;
top: 0;
background: rgb(255, 255, 255); /* The Fallback */
background: rgba(255, 255, 255, 1);
z-index: 10;
height: 300px;
width: 770px;
}
#content-box {
margin-top: 300px;
}
You can copy the file from this Pastebin link too. http://pastebin.com/5dhAeLjq
This assumes you use the default layout which is 770px wide.
This will make the header and all it's elements have a fixed position, have a solid white background, and will be on top of other divs and elements.
The content-box will start with a 300px margin on top. This matches the height of the header.
If you create that one file wp-content/themes/coraline-child/style.css and then activate the new "Coraline Child Theme", then you may get the effect you are looking for.