Named Styles contain wrong xfId
*Created originally on Bitbucket by [Stephan Richter](https://bitbucket.org/%7B0611c01d-7a2e-4bc0-be6a-2963f88b6d15%7D/)*
After loading a workbook, all named styles are available. But none of the styles gets applied correctly. This is due to the fact that the parser code assigns a new fxId based on the index of the named styles instead of honoring the fxId that is stored in the XML.
The bug can be found in `openpyxl.styles.named_styles`, line 288, where is says:
```python
style._set_index(len(styles)) # assign xfId
```
Instead it should be:
```python
style._set_index(ns.xfId) # assign xfId
```
Unfortunately I do not know enough about hg to make a quick PR. :disappointed:
issue