/**
* Get a <b>copy</b> of the currently used CTFill, if none is used, return a new instance.
*/
private CTFill getCTFill(){
CTFill ct;
if(_cellXf.getApplyFill()) {
int fillIndex = (int)_cellXf.getFillId();
XSSFCellFill cf = _stylesSource.getFillAt(fillIndex);
ct = (CTFill)cf.getCTFill().copy();