Стилі

<Window.Resources>

     <Style x:Key="GreenAndYellow">

          <Setter Property="Control.Background" Value="Yellow" />

          <Setter Property="Control.Foreground" Value="Green" />

          <Setter Property="Control.Margin" Value="5" />

          <Setter Property="Control.BorderThickness" Value="10" />

          <Setter Property="Control.BorderBrush" Value="Red" />

          <Setter Property="Control.FontFamily" Value="Verdana" />

          <Setter Property="Control.FontSize" Value="50"/>

     </Style>

</Window.Resources>

<Button x:Name="button1" Content="1" Style="{StaticResource GreenAndYellow}" />

<StackPanel.Resources>

     <LinearGradientBrush x:Key="GradientBrushForButton" StartPoint="0,0" EndPoint="1,0">

          <GradientStop Color="Blue" Offset="0.2" />

          <GradientStop Color="Yellow" Offset="0.8" />               

     </LinearGradientBrush>

     <Style TargetType="Label">  <!--  Усі мітки -->

          <Setter Property="FontStyle" Value="Normal" />

          <Setter Property="FontFamily" Value="Ariel" />

     </Style> 

</StackPanel.Resources>

<Label>Мітка</Label>

<Button Background="{DynamicResource GradientBrushForButton}">Кнопка</Button>

Розширити вже існуючий стиль

<Style x:Key="NamedStyle" TargetType="TextBox" BasedOn="{StaticResource {x:Type TextBox}}">  

     <Setter property="Opacity" value="0.5" /> 

</Style>

Стиль для віх вікон робимо в App.xaml

<Application.Resources>

      ...

</Application.Resources>

ResourceDictionary - коли стилів багатенько