BUG/ENH: Translate CSS border properties for `Styler.to_excel`
- [x] checks an item on #42276 ("Border styles bug")
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Hello @tehunter! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:











There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-02-10 13:27:41 UTC @github-actions pre-commit Any idea what could be causing the failure? It's something related to filesystems in `test_arrowparquet_options`, which I haven't touched. Looking really good, especially the style round trip. Just one or two comments for optimisation. @github-actions pre-commit needs a whats new 1.5.0, (maybe 1.4.1 if it can be backported).

Otherwise looks good to me, pending we get green tests.

Note that you could have considered using a pattern like:
```
expand_border_left = functools.partial(_border_expander, str="left")
```
and you wouldn't have needed to code your own function wrapper for border expand.

 > can you show a test that exerices this at a high level as well

Its difficult to test this since you have to write to excel and then read that the file rendered correctly.

The round trip tests I requested in `test_style.py` now do this: explicitly they check that:

a) The styled dataframe (with a border) is different to the unstyled dataframe (without border)
b) that the excel file created is read in correctly by openpyxl and has the border styles as expected. > needs a whats new 1.5.0, (maybe 1.4.1 if it can be backported).

Do I write the entry or does a maintainer do that? I propose 2 entries along the lines of:

- Bug Fix: Fixed bug in `CSSToExcelConverter` leading to `TypeError` when border color provided without border style for `xlsxwriter` engine (GH42276)
- Enhancement / Styler Enhancements: Added the ability to render `border` and `border-{side}` CSS properties in Excel (GH42276) @tehunter u add them looks fine (but before i resolved i think the doc-build was failing) so let's check that. yeah these are failing: https://github.com/pandas-dev/pandas/runs/4841791532?check_suite_focus=true @github-actions pre-commit @tehunter if you get a chance to merge with main, i can take a look at the CI errors you are getting thanks @tehunter 