feat: make UrsaView base.

src/Ursa.Themes.Semi/Styles/UrsaView.axaml

@@ -0,0 +1,14 @@
+<Styles xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:u="https://irihi.tech/ursa">
+    <Design.PreviewWith>
+        <Border Padding="20">
+            <!-- Add Controls for Previewer Here -->
+        </Border>
+    </Design.PreviewWith>
+
+    <!-- Add Styles Here -->
+    <Style Selector=":is(u|UrsaView)">
+        <Setter Property="Theme" Value="{DynamicResource {x:Type u:UrsaView}}"/>
+    </Style>
+</Styles>

src/Ursa.Themes.Semi/Styles/_index.axaml

@@ -10,5 +10,6 @@
     <StyleInclude Source="Skeleton.axaml" />
     <StyleInclude Source="ToolBar.axaml"/>
     <StyleInclude Source="TimeBox.axaml"/>
+    <StyleInclude Source="UrsaView.axaml" />
     <!--  Add Styles Here  -->
 </Styles>