ux: change text selection brush to improve readability (#825)

src/Resources/Styles.axaml

@@ -4,6 +4,7 @@
         xmlns:vm="using:SourceGit.ViewModels"
         xmlns:c="using:SourceGit.Converters"
         xmlns:ae="using:AvaloniaEdit"
+        xmlns:aee="using:AvaloniaEdit.Editing"
         xmlns:aes="using:AvaloniaEdit.Search">
   <Design.PreviewWith>
     <StackPanel Orientation="Vertical">
@@ -580,6 +581,19 @@
     </Style>
   </Style>
 
+  <Style Selector="aee|TextArea">
+    <Setter Property="SelectionBorder">
+      <Setter.Value>
+        <Pen Brush="{DynamicResource SystemAccentColor}" Thickness="1" />
+      </Setter.Value>
+    </Setter>
+    <Setter Property="SelectionBrush">
+      <Setter.Value>
+        <SolidColorBrush Opacity="0.5" Color="{DynamicResource SystemAccentColor}" />
+      </Setter.Value>
+    </Setter>
+  </Style>
+
   <Style Selector="aes|SearchPanel">
     <Setter Property="Template">
       <ControlTemplate TargetType="aes:SearchPanel">