I believe you are missing something here because it is not clear to me whether you want to center all page in the middle of the screen or not!!
if so, then go to your Admin cp and on the left panel find Styles & Templates/ Style Manager/
on your style, choose Style Variable Editor and click GO
Then find in the search, doc_margin
you find options to enter, so to center all pages add these values:
Units %
Use the 'top' value for all sides No
Top 0
Right auto
Bottom 0
Left auto
save it and you're done!!
Thanks friend, I do not want to center page, I want the page to occupy 100% in doc_width, the background occupies 100%, but the content that is on the bottom, right and left, 100 px trespasses, for to add a vertical banner there.