Several APIs and settings influence the background color which will be used for spreadsheet
cells when exporting to Excel/OpenOffice formats using 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