You can change the tab font by adding custom CSS in App Admin → Settings → Custom CSS.
To change the font for tab titles:
.easytabs-container .easytabs-tab-item a,
.easytabs-container .easytabs-accordion-item a {
font-family: 'Twentieth Century', sans-serif !important;
}
To change the font for tab content:
.easytabs-container .easytabs-contents .easytabs-content-holder * {
font-family: 'Twentieth Century', sans-serif !important;
}
Replace 'Twentieth Century' with the name of the font you want to use.