Add missed brushes for Fluent theme

src/Avalonia.Themes.Fluent/Accents/BaseDark.xaml

@@ -138,6 +138,9 @@
     <SolidColorBrush x:Key="SystemControlTransparentBrush" Color="Transparent" />
     <SolidColorBrush x:Key="SystemControlErrorTextForegroundBrush" Color="{StaticResource SystemErrorTextColor}" />
     <SolidColorBrush x:Key="SystemControlTransientBorderBrush" Color="#000000" Opacity="0.36" />
+    <SolidColorBrush x:Key="SystemControlHighlightListLowRevealBackgroundBrush" Color="{StaticResource SystemListMediumColor}" />
+    <SolidColorBrush x:Key="SystemControlHighlightListMediumRevealBackgroundBrush" Color="{StaticResource SystemListMediumColor}" />
+    <SolidColorBrush x:Key="SystemControlHighlightAccentRevealBackgroundBrush" Color="{StaticResource SystemAccentColor}" />
     <!-- TODO implement AcrylicBrush -->
     <!--<AcrylicBrush x:Key="SystemControlTransientBackgroundBrush" BackgroundSource="HostBackdrop" TintColor="{StaticResource SystemChromeAltHighColor}" TintOpacity="0.8" FallbackColor="{StaticResource SystemChromeMediumLowColor}" />-->
     <SolidColorBrush x:Key="SystemControlTransientBackgroundBrush" Color="{StaticResource SystemChromeMediumLowColor}" />

src/Avalonia.Themes.Fluent/Accents/BaseLight.xaml

@@ -138,6 +138,9 @@
     <SolidColorBrush x:Key="SystemControlTransparentBrush" Color="Transparent" />
     <SolidColorBrush x:Key="SystemControlErrorTextForegroundBrush" Color="{StaticResource SystemErrorTextColor}" />
     <SolidColorBrush x:Key="SystemControlTransientBorderBrush" Color="#000000" Opacity="0.14" />
+    <SolidColorBrush x:Key="SystemControlHighlightListLowRevealBackgroundBrush" Color="{StaticResource SystemListMediumColor}" />
+    <SolidColorBrush x:Key="SystemControlHighlightListMediumRevealBackgroundBrush" Color="{StaticResource SystemListMediumColor}" />
+    <SolidColorBrush x:Key="SystemControlHighlightAccentRevealBackgroundBrush" Color="{StaticResource SystemAccentColor}" />
     <!-- TODO implement AcrylicBrush -->
     <!--<AcrylicBrush x:Key="SystemControlTransientBackgroundBrush" BackgroundSource="HostBackdrop" TintColor="{StaticResource SystemChromeAltHighColor}" TintOpacity="0.8" FallbackColor="{StaticResource SystemChromeMediumLowColor}" />-->
     <SolidColorBrush x:Key="SystemControlTransientBackgroundBrush" Color="{StaticResource SystemChromeMediumLowColor}" />