There doesn't appear to be a mechanism to retrieve a named style from an HSSFWorkbook, nor is there a way to create a named style for a worksheet. I expected that since such convenience methods were not available on the HSSFWorksheet, I could find a getName() method on the HSSFCellStyle class. Unfortunately it wasn't there either. Quickly searching through the 2.5.1 POI codebase, it appears that although the StyleRecord records reference ExtendedFormatRecord records, no effort is made to tie these together and provide users a mechanism to access styles by name, much less create them [by name].
Wanna provide a patch :)
User-defined cell styles are supported in POI-3.8-beta3 Yegor