Fix faint mouseover effect on flat buttons (#1622) (#1690)

Author: AmRo045

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Button.xaml

@@ -151,7 +151,7 @@
                                 CornerRadius="{Binding Path=(wpf:ButtonAssist.CornerRadius), RelativeSource={RelativeSource TemplatedParent}}"
                                 BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
                             <Border.Resources>
-                                <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{DynamicResource PrimaryHueMidBrushColor}" Opacity="0.04"/>
+                                <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{DynamicResource PrimaryHueMidBrushColor}" Opacity="0.16"/>
                             </Border.Resources>
                             <wpf:Ripple Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" Focusable="False"
                                         HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
@@ -221,7 +221,7 @@
                                 BorderBrush="{TemplateBinding BorderBrush}"
                                 BorderThickness="{TemplateBinding BorderThickness}">
                             <Border.Resources>
-                                <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{DynamicResource PrimaryHueMidBrushColor}" Opacity="0.04"/>
+                                <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{DynamicResource PrimaryHueMidBrushColor}" Opacity="0.16"/>
                             </Border.Resources>
                             <wpf:Ripple Content="{TemplateBinding Content}" 
                                         ContentTemplate="{TemplateBinding ContentTemplate}"