public interface ExportBGColor
ListGrid.exportData() or ListGrid.exportClientData().  The
 following APIs are called in the order  shown, so hilite.backgroundColor takes
 precedence over  exportDefaultBGColor, for example. 
 If overriding any of the above methods, return null to allow methods later in the precedence
 order to influence background color.  For example, if you want certain rows to have a special
 background color but also want to show alternating colors per row, override getExportRowBGColor
 and return null for all rows that should just show normal alternating colors, and not a special
 color.