fix: add CSS layer to bypass base rules

[hugoattal]

Description

I added a CSS layer to make it easier to overwrite the base CSS.

Additional context

Ref:

• https://developer.mozilla.org/fr/docs/Web/CSS/@layer
• https://caniuse.com/mdn-css_at-rules_layer

Note, I changed the layer @base to @histoire: it seems that this specific name for a layer does weird stuff with PostCSS.

⚠️ I have no idea what is the problem with the workflow... I even revert the pnpm-lock file (which is a bit outdate, we're at version 9 of pnpm 😕)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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