Hello Fausto, 

You can only probably achieve  using another font in Amelia by adding custom CSS. 

Yes you try with this format HTML with which you will overwrite font in Amelia:

<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 provided font fantasy just as an example, but you can provide the font that you want there.