I do not help with CSS. Only 2 Volunteers do and they do not appear to be logged in and posting. I did a forum search for you on Depo Masthead custom header. here are the results http://tinyurl.com/6jhrgj8
Ummm if you would like to design your own header and try it out then wordpress.com has provided a preview function so you can try before you buy here > Appearance > Edit CSS
If you have CSS editing experience, you can get the paid custom CSS upgrade and renew it annually. It will allow you to stylize the appearance of themes you find here > Appearance > Themes, but it does not allow you to change the functionality, by editing the underlying template. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own.
If you do purchase the upgrade be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/
Atff have also provided these resources and some advice > http://en.support.wordpress.com/custom-css/#css-help