Change background opacity on startup

Question

I would like to change background opacity (make it darker) when programs starts

<Window x:Class="PCSX4.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:PCSX4"
        mc:Ignorable="d"
        Title="MainWindow" Height="900" Width="900"
        WindowStyle="None"
        WindowStartupLocation="CenterScreen"
        Background="Transparent">

    <WindowChrome.WindowChrome>
        <WindowChrome GlassFrameThickness="0,0,0,1" CornerRadius="0" />
    </WindowChrome.WindowChrome>

    <Grid>

        <Border Background="lightBlue" />
        <Image HorizontalAlignment="Center" Height="1002" VerticalAlignment="Center" Width="900" RenderTransformOrigin="-2.3,-0.484" Source="/background.png" Stretch="Fill"/>

    </Grid>

</Window>

Haven't tried anything because I don't know how to solve this problem as I'm new to this.