feat: neutral theme -- option to override DSFR

[edelagnier]

Apply a distinct CSS when the Marianne option is false to allow usage outside of the french government.

• Replace the Marianne font by Helvetica
• Change color scheme (blue -> green) to ensure a notable visual change. Could be improved upon later to create a dedicated theme.

An example of the result can be seen here : Dashlord

[sonarcloud[bot]]

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[revolunet]

Trop bien merci 👍

[github-actions[bot]]

:tada: This PR is included in version 1.32.0 :tada:

The release is available on GitHub release

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