REF: refactor/cleanup of CSSToExcelConverter
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Refactor/clean up of ``CSSToExcelConverter`` in module ``pandas.io.formats.excel``.

- Move class variables to the top of the class
- Add font family mapping
- Extract methods
- Add missing typing
- Make color parsing cleaner