WPF Add ResourceDictionary file and declared in app.xaml

//Add resource dictionary file  named Brushes.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <LinearGradientBrush EndPoint="1,0" x:Key="brush1">
        <GradientStop Color="Yellow" Offset="0"/>
        <GradientStop Color="Orange" Offset="0.7"/>
        <GradientStop Color="DarkRed" Offset="1"/>
    </LinearGradientBrush>
</ResourceDictionary>



//Declared in the App.xaml  via ResourceDictionary.MergedDictionary so that it can available for the whole project

//App.xaml
<Application x:Class="WpfApp15.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp15"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Resources/Brushes.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>


//Main.xaml

<Window x:Class="WpfApp15.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp15"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Ellipse Fill="{StaticResource brush1}"/>
    </Grid>
</Window>

The key located at declaration the newly added resource dictionary file via ResourceDctionary.MergedDictionary so it will be accessed in the project

//App.xaml
<Application x:Class="WpfApp15.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp15"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Resources/Brushes.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>