Centre Header Image

Hello,
I would like to center the Header Image and have gone through your forums but cannot get it right. Are you able to assist.

I tried copying in the following code within the Custom CSS section, without success:
#site-logo, hgroup.with-logo {
clear: none;
display: inline;
float: centre;
}

My site is http://www.grahamgouldmotors.com.au

I have managed to find a way since but not sure if it is centred correctly. Also, is there a way to change the colour around the image. Sorry one more thing centering the header text as well.

Thanks,
Regards,
Anthony