fix custom notification styling

src/Avalonia.Themes.Default/NotificationArea.xaml

@@ -1,4 +1,5 @@
-<Styles xmlns="https://github.com/avaloniaui">
+<Styles xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
     <Style Selector="NotificationArea">
         <Setter Property="Margin" Value="0 0 8 8"/>
         <Setter Property="Template">
@@ -6,11 +7,14 @@
                 <StackPanel Name="PART_Items">
                     <StackPanel.DataTemplates>
                         <DataTemplate DataType="NotificationContent">
-                            <StackPanel Spacing="8">
+                            <StackPanel Spacing="8" Margin="12">
                                 <TextBlock DockPanel.Dock="Top" Text="{Binding Title}" FontWeight="Medium" />
-                                <TextBlock MaxHeight="80" Text="{Binding Message}" TextWrapping="Wrap" Opacity=".8" Margin="0,0,12,0"/>
+                                <TextBlock MaxHeight="80" Text="{Binding Message}" TextWrapping="Wrap" Margin="0,0,12,0"/>
                             </StackPanel>
                         </DataTemplate>
+                        <DataTemplate DataType="x:String">
+                            <TextBlock Text="{Binding }" Margin="12" />
+                        </DataTemplate>
                     </StackPanel.DataTemplates>
                 </StackPanel>
             </ControlTemplate>
@@ -29,8 +33,7 @@
                     <Border Background="{TemplateBinding Background}" 
                             BorderBrush="{TemplateBinding BorderBrush}" 
                             BorderThickness="{TemplateBinding BorderThickness}"
-                            Margin="8,8,0,0"
-                            Padding="12">
+                            Margin="8,8,0,0">
                         <ContentControl MinHeight="150" Content="{TemplateBinding Content}" />
                     </Border>
                 </LayoutTransformControl>
@@ -99,7 +102,10 @@
         <Setter Property="HorizontalAlignment" Value="Right"/>
         <Setter Property="VerticalAlignment" Value="Top"/>
     </Style>
-    
+
+    <Style Selector="Notification">
+        <Setter Property="Background" Value="#444444"/>
+    </Style>
     <Style Selector="Notification:information">
         <Setter Property="Background" Value="Teal"/>
     </Style>