<UserControl
    x:Class="ExperimentsWpf.UserControl1"
    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"
    Width="100"
    Height="100"
    d:DesignHeight="450"
    d:DesignWidth="800"
    Background="Red"
    mc:Ignorable="d" />

<Window
    x:Class="ExperimentsWpf.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:ExperimentsWpf">
    <Window.Resources>
        <Style x:Key="StyleUserControl1" TargetType="{x:Type local:UserControl1}">
            <Setter Property="Width" Value="10" />
            <Setter Property="Height" Value="10" />
            <Setter Property="Background" Value="Green" />
        </Style>
    </Window.Resources>
    <Grid VerticalAlignment="Center">
        <local:UserControl1 Style="{StaticResource StyleUserControl1}" />
    </Grid>
</Window>

<UserControl
    x:Class="ExperimentsWpf.UserControl1"
    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"
    d:DesignHeight="450"
    d:DesignWidth="800"
    mc:Ignorable="d">
    <UserControl.Style>
        <Style TargetType="UserControl">
            <Setter Property="Width" Value="100"/>
            <Setter Property="Height" Value="100"/>
            <Setter Property="Background" Value="Red"/>
        </Style>
    </UserControl.Style>
</UserControl>