Yes, it's definitely possible. It looks like most of that white space is the margin around the navigation menu, so you will want to adjust this piece of code:
#access {<br />
margin: 1.615em auto;<br />
}
The way it is written, the first number controls the margins above and below the menu, while the "auto" value controls the side margins. If you only want to change the white space above the menu without changing the space below it, use this code:
#access {<br />
margin: 1.615em auto 1.615em;<br />
}
The first value is the top margin, the second is the side margins, and the third is the bottom margin. (You can read more about how this works in this W3Schools CSS Margin article.)
I really encourage you to look over the CSS Basics page to get a better sense of how CSS works. It will help you learn to explore this code and feel more confident working with it.
The Firebug add-on for Firefox is my favorite CSS tool, because you can right-click on any element on your website and inspect the code behind it. You can also test out changes to the code and see the results on the screen without changing how anyone else views your website. I find that very helpful!