Using ResourceDictionary

First, lets create one XAML with styles:

<Window x:Class="ResourceDictionary.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

    <Window.Resources>
        <Style x:Key="myStyle" TargetType="Button">
            <Setter Property="Background" Value="Orange" />
            <Setter Property="FontStyle" Value="Italic" />
            <Setter Property="Padding" Value="8,4" />
            <Setter Property="Margin" Value="4" />
        </Style>
    </Window.Resources>
    
    <Grid>
        <StackPanel Orientation="Horizontal" VerticalAlignment="Top">
            <Button Style="{StaticResource myStyle}">Styles</Button>
            <Button Style="{StaticResource myStyle}">are</Button>
            <Button Style="{StaticResource myStyle}">cool</Button>
        </StackPanel>
    </Grid>
</Window>

So, first part which you have to notice is:

<Window.Resources>
	<Style x:Key="myStyle" TargetType="Button">
		<Setter Property="Background" Value="Orange" />
		<Setter Property="FontStyle" Value="Italic" />
		<Setter Property="Padding" Value="8,4" />
		<Setter Property="Margin" Value="4" />
	</Style>
</Window.Resources>

With that part we are defining resources, and to use it, we can simply write something like:

Style="{StaticResource myStyle}"

Notice myStyle, this is name of our resource.

Now, lets move that our style to ResourceDictionary.

Start new WPF project, in solution explorer right click on the project add -> Resource Dictionary:

Name it as you wish (in my case I name it Dictionary1.xaml), now cut myStyle from MainWindow.xaml and paste it to your dictionary file, in my case it looks like this:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style x:Key="myStyle" TargetType="Button">
        <Setter Property="Background" Value="Orange" />
        <Setter Property="FontStyle" Value="Italic" />
        <Setter Property="Padding" Value="8,4" />
        <Setter Property="Margin" Value="4" />
    </Style>
</ResourceDictionary>

In MainWindow.xaml write something like:

<Window x:Class="ResourceDictionary.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary 
                  Source= "Dictionary1.xaml">
                </ResourceDictionary>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>

    <Grid>
        <StackPanel Orientation="Horizontal" VerticalAlignment="Top">
            <Button Style="{StaticResource myStyle}">Styles</Button>
            <Button Style="{StaticResource myStyle}">are</Button>
            <Button Style="{StaticResource myStyle}">cool</Button>
        </StackPanel>
    </Grid>
</Window>

Note source, and that is all.