Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS makes fonts bigger on the mobile version.
* 11329777-hc (jw)
*/
@media screen and (max-width: 782px) {
body {
font-size: 100%;
line-height: 1.6;
}
}
Tip: keep the /*
comment */
in the code to later know what this code does.