SMART datagrid v.1 > Classes > ColumnHeader
Specify as the distance between check box or image being displayed in header cell and text. If this value is undefined, it will use GridHeader.itemGap.
    column.setHeader({
        itemGap: 4,
        itemOffset: 2
    });
    
    // or, set individually.
    column.header().setItemGap(4);
    
    // can also use setProperty.
    column.setProperty('header.itemGap', 4);