There's lots and lots of advice on how to use a custom theme. I have the opposite problem: Up until recently, all my hosted doc sites used the gorgeous read-the-docs theme by default because that's what happens when you do not pick a theme. But for some reason, sophie.... has gotten hung up on the Alabaster theme and I can't figure out how to switch it back.
One vaguely-related question had a suggestion to delete temporary files, but (a) this is quite vague, and (b) I don't think it makes sense now that RTD uses containerized builds. Another suggestion was to make an explicit dependency on the official RTD theme, but for whatever reason that's not plug-and-play on my local machine.
To be clear, a full doc rebuild succeeds without error. It just astonishingly chooses the Sphinx default theme instead of the RTD default theme.
I may once have accidentally had the Alabaster theme set explicitly, but I've commented it out. But then I did that on other documentation projects and they're all on the default theme just fine. As far as I can tell, the configuration is identical in all respects between projects A and B.
So it turns out https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html has the answer. There is no need to futz with project dependencies. Just the conf.py edits are sufficient to make this work. Paraphrasing:
extensions = [ "sphinx_rtd_theme", ]
html_theme = "sphinx_rtd_theme"
You will need to pip install sphinx_rtd_theme to build docs locally, but that package is installed by default in the system image that ReadTheDocs now builds with.