MyBBThemes Community

I want to convert my current SMF forum to myBB. I like my current SMF Theme (made by a friend). I used to play with SMF themes and now myBB seems little bit different.
One problem that I encountered is the header
I want the header wider than the main body, So I am asking if I can have div header outside of wrapper div

Err... I think you can with every forum software, and with simple HTML/CSS, too but I have no idea what the code would be.