feat: add default Theme to SplashWindow.

demo/Ursa.Demo/Views/MainSplashWindow.axaml

@@ -2,22 +2,11 @@
     x:Class="Ursa.Demo.Views.MainSplashWindow"
     xmlns="https://github.com/avaloniaui"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:iri="https://irihi.tech/shared"
-    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     xmlns:u="https://irihi.tech/ursa"
     Title="MainSplashWindow"
     Width="400"
-    Height="400"
-    d:DesignHeight="450"
-    d:DesignWidth="800"
-    CountDown="0:0:3"
-    ExtendClientAreaChromeHints="NoChrome"
-    ExtendClientAreaTitleBarHeightHint="0"
-    ExtendClientAreaToDecorationsHint="True"
-    SystemDecorations="None"
-    WindowStartupLocation="CenterScreen"
-    mc:Ignorable="d">
+    Height="400">
     <Grid
         HorizontalAlignment="Center"
         VerticalAlignment="Center"
@@ -51,4 +40,4 @@
                 Text="Aesthetic Evolution of Productivity" />
         </StackPanel>
     </Grid>
-</u:SplashWindow>
+</u:SplashWindow>