You probably need to duplicate and update your CSS rules using a media query, so they get applied on smaller screens.
Say you have this normally:
<style>
.content { /* your font css code */ }
</style>
Then you'll need to add something like this and update any classes/ids if they are different on mobile:
<style>
@media screen and (min-width: 480px) {
.content {
/* font code */
}
}
</style>
