[Theme:Pinboard] Center Header Logo

Hey mates!

I searched in this forum for help with the Pinboard theme and seems that many people have problems to center the header image of this theme.
this is the site I am currently working on:
http://massagearahant.nl/wordpress/

I tried:
#site-title img {
margin-left: auto;
margin-right: auto;
}

like pail.a.cunn described in this thread: http://wordpress.org/support/topic/center-header-image-6

however this does not work… and if I set the margins on 50% it does not center in the middle and also does not float with the rest of the theme.

Any other suggestions?