Not sure if this will work but try changing

.skinheader
{
text-align: center;
}

and see what happens. The default skin has the table setup to align left for the logo field, but it calls the .skinheader class for formatting the content so that might help. Can't promise though.  In general you would be better off making a copy of the default skin and then making changes to the table where you see [LOGO] token and set it to align: center  then upload the skin and install it as the new skin.

The default skin does, but there is nothing stopping you from creating a tablelless skin....