TST: Increase test coverage for pandas.io.formats.excel.py
Sorry for the appauling number of commits. Git was being unkind, and I accidentally pushed .venv/ to gitignore, which then when I tried to revert, would reinclude the venv on the commit, you get the picture. It won't happen again.


The purpose of this PR is to add some test coverage to the first class of pandas.io.formats.excel.py, in the CSSToExcelConverter class. This class is missing some coverage, and there were a few unused functions and lines of code that were also causing some code coverage problems, so they have been dealt with. 
<img width="1345" alt="Screenshot 2025-06-23 at 6 58 27 PM" src="https://github.com/user-attachments/assets/9610b4c6-ed5f-4323-9a9b-76de3ae0dcbd" />

<img width="1338" alt="Screenshot 2025-06-23 at 6 59 14 PM" src="https://github.com/user-attachments/assets/11a159de-3ca4-40a9-ade0-49347cf12157" />

<img width="1303" alt="Screenshot 2025-06-23 at 6 59 24 PM" src="https://github.com/user-attachments/assets/e3f967c1-9ef4-4927-b070-1c8af9ba00d1" />

These screenshots show the areas I am adding coverage too.
Thanks @lsgordon 