who knows xhtml or has zen cart?

i'm reworking a bunch of different things about my website and have just one tiny thing i can't seem to work out - do you have any idea how to make my header centered horizontally and vertically?

here's where to look:
http://www.heartsandlaserbeams.com/store

and here's the code i need to change to make it work - it's in xhtml for your reference:

// added defines for header alt and text
define('HEADER_ALT_TEXT', 'hearts and laserbeams hearts you!');
define('HEADER_SALES_TEXT', ' ');
define('HEADER_LOGO_WIDTH', '192px');
define('HEADER_LOGO_HEIGHT', '64px');
define('HEADER_LOGO_IMAGE', 'logo.gif');

thanks for your help if you can help!

steph!