teach XSSFColor and most things that create instances about indexed colors.
Null is a valid value for IndexedColorMap instances - the existing built-in default colors are used.
Whenever a workbook style is accessible in the call hierarchy its color mappings are passed down now.
Thanks for the unit test in the issue, it now passes.
git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1796359 13f79535-47bb-0310-9956-ffa450edef68
