Yes by using a custom CSS theme.
First, if you don't already have a theme, create one via File > CSS Themes.
Second, be sure your app is using that theme by checking File > App Settings > CSS Theme.
The following example imports a custom font and then applies it to all tab widgets in the app.
/* import a custom font */
@import url('https://fonts.googleapis.com/css?family=Srisakdi');
/* make all TabsWidgets use that font */
div.TabsWidget {
font-family: Srisakdi;
font-size: 13px;
}
/* to only impact those tabs with their Format>Custom CSS Class set to Funky use this instead */
div.TabsWidget.Funky {
font-family: Srisakdi;
font-size: 13px;
}