Castania,
learning from the theme's CSS is a good start
https://github.com/sphinx-doc/alabaster/blob/master/alabaster/static/alabaster.css_t#L63
Make additions and replacements as desired in the `custom.css` file on the `static` folder of the template.
The keywords you're looking for are "how to customise Sphinx themes" or "Sphinx custom CSS" as in https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html
Cheers,
Jayson