code: public void setContentCentered(boolean contentCentered) {
          boolean oldVal = this.contentCentered;
          this.contentCentered = contentCentered;
          if (oldVal != this.contentCentered && content != null && content.isAttached()) {
              if (this.contentCentered) {
                  centerContent();
                  initWindowResize();
              } else {
                  clearCenterContent();
              }
          }
      }
docs: Content band will be centered between Header and Footer
  (they must be defined as fixed="true" OR page.pseudoFixedToolbars="true").