Created attachment 32862 [details]
Test case see Main.java

In attachment a test case for this bug (see Main.java)

Basically :

1) I create a "reference" Workbook from a file "template.xlsx" that contains only 1 custom style;
2) I create a "target" Workbook (empty)
3) I copy the custom style from the "reference" workbook to the "target" workbook
4) I create one cell with the custom style
5) I output the target Workbook to a file
6) I close everyting and open the output file with Excel and I get an error about "styles.xml" corrupted

For some custom the style in "template.xlsx" I can get a output file without error, but with the one in the attached zip, it fails everytimes.