Here's the code for the left rectangle... (and for reason, you have to click improve solution to see the rest of the code)
<window x:class="MainWindow" xmlns:x="#unknown">
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:PrioKat"
mc:Ignorable="d"
Title="PrioKat" Height="463.309" Width="673.355" BorderThickness="1" ResizeMode="CanMinimize" AllowsTransparency="True" WindowStyle="None">
<window.background>
<lineargradientbrush endpoint="0.5,1" startpoint="0.5,0">
<gradientstop color="Black" offset="0" />
<gradientstop color="#FF262344" offset="1" />
</lineargradientbrush>
</window.background>
<rectangle fill="#FF08081D" horizontalalignment="Left" margin="-4,0,0,1" stroke="Black" width="151" />
</window>
And this is the right rectangle...
<window x:class="MainWindow" xmlns:x="#unknown">
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:PrioKat"
mc:Ignorable="d"
Title="PrioKat" Height="463.309" Width="673.355" BorderThickness="1" ResizeMode="CanMinimize" AllowsTransparency="True" WindowStyle="None">
<window.background>
<lineargradientbrush endpoint="0.5,1" startpoint="0.5,0">
<gradientstop color="Black" offset="0" />
<gradientstop color="#FF262344" offset="1" />
</lineargradientbrush>
</window.background>
<rectangle fill="#FF08081D" horizontalalignment="Left" margin="-4,0,0,1" stroke="Black" width="151" />
</window>