DOC: improve (dark) theme getting started tutorials
- [x] closes #51860 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This PR updates thes CSS relevant for the getting started section  of the documentation to make sure the available css-variables of the pydata-sphinx-theme are used instead of custom css-colors. As such, the pages will also work in dark-mode (as colors are adjusted based on the used variable names).


/preview Website preview of this PR available at: https://pandas.pydata.org/preview/52097/