CSS variables in conf.py being ignored (with Sphinx 4)

[janluke]

I upgraded furo from version 2021.4.11b34 to 2021.6.18b36. All CSS variables overridden in conf.py are ignored now.

You can find my conf.py here: https://github.com/janluke/cloup/blob/master/docs/conf.py
You can compare my release and latest docs to see that the brand color is different despite the fact I've not changed it:
https://cloup.readthedocs.io/en/stable/
https://cloup.readthedocs.io/en/latest/

UPDATE: it looks like the problem exists with Sphinx 4 only.

[janluke]

Duplicated of #162. Sorry.