Added slight translucency for better look of flat buttons in dark style

Author: mgnslndh

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Dark.xaml

@@ -18,7 +18,7 @@
     <SolidColorBrush x:Key="MaterialDesignDivider" Color="#1FFFFFFF" po:Freeze="True" /><!-- 12% -->
     <SolidColorBrush x:Key="MaterialDesignSelection" Color="#757575" po:Freeze="True" />
 
-    <SolidColorBrush x:Key="MaterialDesignFlatButtonClick" Color="#757575" po:Freeze="True" />
+    <SolidColorBrush x:Key="MaterialDesignFlatButtonClick" Color="#19757575" po:Freeze="True" /> <!-- 25% -->
     <SolidColorBrush x:Key="MaterialDesignFlatButtonRipple" Color="#FFB6B6B6" po:Freeze="True" />
 
     <SolidColorBrush x:Key="MaterialDesignToolTipBackground" Color="#eeeeee" po:Freeze="True" />