Content/UserControl styles were missing.

src/Perspex.Themes.Default/DefaultTheme.paml

@@ -4,6 +4,7 @@
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.Button.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.Carousel.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.CheckBox.paml"/>
+  <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.ContentControl.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.DropDown.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.GridSplitter.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.ItemsControl.paml"/>
@@ -23,5 +24,6 @@
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.ToggleButton.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.TreeView.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.TreeViewItem.paml"/>
+  <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.UserControl.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.Window.paml"/>
 </Styles>

src/Perspex.Themes.Default/UserControl.paml

@@ -0,0 +1,13 @@
+<Style xmlns="https://github.com/perspex" Selector="UserControl">
+  <Setter Property="Template">
+    <ControlTemplate>
+      <Border Background="{TemplateBinding Background}"
+              BorderBrush="{TemplateBinding BorderBrush}"
+              BorderThickness="{TemplateBinding BorderThickness}">
+        <ContentPresenter Name="PART_ContentPresenter" 
+                          Content="{TemplateBinding Content}" 
+                          Margin="{TemplateBinding Padding}"/>
+      </Border>
+    </ControlTemplate>
+  </Setter>
+</Style>