Click the Settings icon and select More Settings. Then, choose Writing email in order to change the default font used in Yahoo Mail.
If your Yahoo Mail account suddenly changed to an ExtraBold Poppins font and you find it horrible, don't worry, there's a way to reset it. First, go to the Settings or Options section of your Yahoo Mail account. Look for the "Appearance" or "Fonts" settings. From there, you should be able to change the font style back to the original one. Just select a different font option that you prefer and save the changes. That should restore your Yahoo Mail to its original font style.