Custom colours not working #8526

Context

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Bug description

So I've followed the steps here:

https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#custom-colors

I have the following for the theme block in my mkdocs.yml:

theme:
  name: material
  palette:
    primary: custom
  extra_css:
    - stylesheets/extra.css
  features:
    - navigation.tabs
    - navigation.sections
    - navigation.expand
    - navigation.top
    - search.suggest
    - search.highlight
    - content.code.annotate
    - content.code.copy

And then use the following in docs/stylesheets/extra.css:

:root > * {
  --md-primary-fg-color: #25252d;
  --md-primary-fg-color--light: #25252d;
  --md-primary-fg-color--dark: #25252d;
  --md-accent-fg-color: #25252d;
}

But I'm always ending up with the default blue colour. Am I missing something obvious here?

Related links

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Reproduction

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Steps to reproduce

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Browser

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