How to reproduce the problem:
- Open new file in Calc
- Enter "ab" in field A1
- Mark A1 and push button "Conditional Formatting: Condition"
- Cell value is equal to "ab"
- Apply style: New Style...
- Background: Select color, e.g.: Green
- Field A1 should now be green
- File - Save As...
- Enter name and pick filter: Microsoft Excel 2007-2013 XML (.xlsx)
- Close LibreOffice
- Open file in LibreOffice: the background of A1 is not green anymore
*** This bug has been marked as a duplicate of bug 99866 ***