flat button font size correction. fixes #340

Author: ButchersBoy

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Button.xaml

@@ -95,6 +95,7 @@
         <Setter Property="Foreground" Value="{DynamicResource PrimaryHueMidBrush}"/>
         <Setter Property="wpf:RippleAssist.Feedback" Value="{DynamicResource MaterialDesignFlatButtonRipple}" />
         <Setter Property="TextBlock.FontWeight" Value="DemiBold"/>
+        <Setter Property="TextBlock.FontSize" Value="14"/>
         <Setter Property="BorderThickness" Value="1"/>
         <Setter Property="HorizontalContentAlignment" Value="Center"/>
         <Setter Property="VerticalContentAlignment" Value="Center"/>