Update RichTextBlock ControlTheme

src/Avalonia.Themes.Fluent/Controls/FluentControls.xaml

@@ -68,6 +68,7 @@
                 <ResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/Slider.xaml" />
                 <!--  ManagedFileChooser comes last because it uses (and overrides) styles for a multitude of other controls...the dialogs were originally UserControls, after all  -->
                 <ResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/ManagedFileChooser.xaml" />
+                <ResourceInclude Source="avares://Avalonia.Themes.Fluent/Controls/RichTextBlock.xaml"/>
             </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>
     </Styles.Resources>

src/Avalonia.Themes.Fluent/Controls/RichTextBlock.xaml

@@ -1,10 +1,14 @@
-<Styles xmlns="https://github.com/avaloniaui"
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
     <Design.PreviewWith>
         <RichTextBlock IsTextSelectionEnabled="True" Text="Preview"/>
     </Design.PreviewWith>
 
-    <Style Selector="RichTextBlock[IsTextSelectionEnabled=true]">
+  <ControlTheme x:Key="{x:Type RichTextBlock}" TargetType="RichTextBlock">
+    <Style Selector="^[IsTextSelectionEnabled=True]">
       <Setter Property="Cursor" Value="IBeam" />
     </Style>
-</Styles>
+  </ControlTheme>
+
+
+</ResourceDictionary>