Hello Yves Recour, 

At the moment, the only two available options for the Font in Amelia -> Customize are the Roboto and Lato. 

But you can try adding this custom HTML to the page where you have your Amelia shortcode on and change the font this way:

<style>
.amelia-app-booking input[type="text"], .am-modal.am-in-body input[type="text"], .amelia-app-booking input[type="email"], .am-modal.am-in-body input[type="email"], .amelia-app-booking input[type="tel"], .am-modal.am-in-body input[type="tel"], .amelia-app-booking input[type="url"], .am-modal.am-in-body input[type="url"], .amelia-app-booking textarea, .am-modal.am-in-body textarea, .amelia-app-booking select, .am-modal.am-in-body select, .amelia-app-booking .form-control, .am-modal.am-in-body .form-control, .amelia-app-booking .am-stripe-card, .am-modal.am-in-body .am-stripe-card,
.amelia-app-booking h1, .am-modal.am-in-body h1, .amelia-app-booking h2, .am-modal.am-in-body h2, .amelia-app-booking h3, .am-modal.am-in-body h3, .amelia-app-booking h4, .am-modal.am-in-body h4, .amelia-app-booking p, .am-modal.am-in-body p, .amelia-app-booking span, .am-modal.am-in-body span, .amelia-app-booking div, .am-modal.am-in-body div, .amelia-app-booking section, .am-modal.am-in-body section, .amelia-app-booking ul, .am-modal.am-in-body ul, .amelia-app-booking li, .am-modal.am-in-body li, .amelia-app-booking a, .am-modal.am-in-body a {font-family:fantasy !important;}
</style>

We set the font to be fantasy just as an example, you can set the font you use on your website. 

Hope this helps!

If you have any further questions or concerns feel free to ask,