To change a font family, use the following CSS code. Navigate to Dashboard > Forums > Settings > Styles admin page, put it in "Custom CSS Code" textarea. 

#wpforo #wpforo-wrap *:not(.fa):not(.fas):not(.far):not(fab) {
    font-family: "Arial Rounded MT Std", Arial, sans-serif !important;
}

Don't forget to change the red marked color code before using.

Font size settings are located in Dashboard > Forums > Settings > Styles admin page.