Checking the source of
https://demo.matomo.cloud/index.php?module=CoreAdminHome&action=optOut&language=fr&backgroundColor=f2f6ff&fontColor=2e2f32&fontSize=14px&fontFamily=apple-system
it contains font-family: apple-system; as requested.
But font-family: apple-system; doesn't really exist, do you by chance mean -apple-system? And even then you might also want to specify another font for non-apple devices.

Hi,
Indeed my initial expectation was to use another font which is "Libre Franklin Regular" (the one used on the tracked site) but it did not work either therefore I tested with a font used by Matomo to confirm the analysis.
Alice

the one used on the tracked site

Keep in mind that the opt out is in an iFrame and therefore doesn't have any access to webfonts or any other content of the parent page. So unless the uses has the font installed or you use https://plugins.matomo.org/CustomOptOut to add more CSS (with the webfont) to the Matomo page, then it is expected that it won't work.

But at this point you might want to instead use https://developer.matomo.org/guides/tracking-javascript-guide#optional-creating-a-custom-opt-out-form to make an opt out form on your website itself instead of an iFrame, which has the advantage of being styled just like any other content on your site.

@Alice-Pa I'll close this issue for now as everything is working as expected on our side. Feel free to continue commenting though.

We're starting the work on issue #17452 very soon. This will be a different way of providing the opt out where it will then be possible (or even the default) that your site's font will be used.