Nitpicky detail: there's a typo in the CSS filename referenced from the Customization section of the Alabaster docs:

Create a file named custom.css anywhere you prefer (typically _static/, but this is solely convention) containing your desired overrides to the CSS found in Alabaster’s static/alabaster_css_t.

I think that's meant to be static/alabaster.css_t, not static/alabaster_css_t.