SMART datagrid v.1 > Classes > ColumnHeader
Specify as the distance between header cell border and header cell internal items. 
If this value is undefined, it will use itemOffset value of GridHeader.
    column.setHeader({
        itemGap: 4,
        itemOffset: 2
    });
    
    // or, set individually.
    column.header().setItemOffset(2);
    
    // can also use setProperty.
    column.setProperty('header.itemOffset', 2);