please help me for center my page
This is not so easy if I recall. I'm not near a PC so I can't verify but I believe the goblue theme is built using bootstraps grid system which makes use of a column system by my of classes on the HTML elements. Of course you could override the bootstrap class with use of custom css via the custom css/js component or you could create a copy of the goblue theme and modify that file to use centered layouts. For example if there are 3 columns then you would balance out the 12 available by doing say col-md-3 on the left, col-md-6 on the middle, and col-md-3 on the right which adds up to the 12 available.