HandyControl/src/Shared/HandyControlDemo_Shared/Resources/Themes/Basic/Fonts.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <FontFamily x:Key="FabricIcons">pack://application:,,,/HandyControlDemo;component/Resources/fabric-icons.ttf#Fabric MDL2 Assets</FontFamily>

</ResourceDictionary>