ENH: Making header_style a property of ExcelFormatter #22758
- [y] closes #22758
- [y] tests added / passed
- [y] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


Hello @dannyhyunkim! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/io/formats/excel.py`](https://github.com/dannyhyunkim/pandas/blob/f30ca4e572de64ca2d5f7165ba27e4afbabf0cec/pandas/io/formats/excel.py) !

 Nice job! Can you add a whatsnew note for 0.25? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=h1) Report
> Merging [#22759](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1a12c41d201f56439510e683fadfed1218ea9067?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `83.33%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22759/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22759      +/-   ##
==========================================
+ Coverage   92.16%   92.17%   +0.01%     
==========================================
  Files         169      169              
  Lines       50769    50770       +1     
==========================================
+ Hits        46791    46799       +8     
+ Misses       3978     3971       -7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <83.33%> (+0.01%)` | :arrow_up: |
| #single | `42.32% <16.66%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/22759/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.4% <83.33%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22759/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.67% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/console.py](https://codecov.io/gh/pandas-dev/pandas/pull/22759/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY29uc29sZS5weQ==) | `75.75% <0%> (+10.6%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=footer). Last update [1a12c41...2f1c6df](https://codecov.io/gh/pandas-dev/pandas/pull/22759?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @WillAyd , wasn't entirely sure if this was what you were after, but I had a crack! This is totally my mistake but I meant v0.24 (a whatsnew should already exist). 

You did a great job in adding one but unfortunately that’s not what we want here. Sorry! No worries at all. I should have clarified earlier.  @WillAyd : Thanks for the ping.  Will take a look later today. Thanks @dannyhyunkim - great first contribution!