help needed to adjust header table and background color

[jaffamaster]

forum http://www.scottishmotocross.co.uk/forum


would like to reduce the size the forum displays to around 1000px, have tried editing within overall_header.html but it only reduces the smaller table that holds the php logo
as you can see here on the test forum http://scottishmotocross.co.uk/forumtest i've a managed to change the background color to white, but can't get the align="center" to work and would like to shrink the whole table.

I've been using the search function, but can't put my finger on the help I need. I'm editing subsilver2, can anyone point to a tutorial to cover these issues?

Thanks

Nick