This depends on where the widget is loaded. If the widget is loaded on the forum page, you should add the following CSS code in Dashboard > Forums > Settings > Styles admin page. If it is on the other pages put the code in Top Black Admin Bar > Customize > "Additional CSS". Don't forget to delete all caches before checking:
#wpforo-wrap .widget-title {
font-family: Arial;
}
Change the red marked color code before using.