add box shadow resource
MODIFY
WalletWasabi.Fluent/Styles/Themes/BaseDark.xaml
WalletWasabi.Fluent/Styles/Themes/BaseDark.xaml
<SolidColorBrush x:Key="TextForeground">#FFFFFF</SolidColorBrush>
<SolidColorBrush x:Key="AcrylicFallbackColor">#1F1F1F</SolidColorBrush>
<Color x:Key="RegionColor">#FF212121</Color>
+ <BoxShadow x:Key="BoxShadow">0 1 4 #121212</BoxShadow>
</Styles.Resources>
<StyleInclude Source="avares://Avalonia.Themes.Fluent/Accents/FluentDark.xaml"/>
</Styles>

MODIFY
WalletWasabi.Fluent/Styles/Themes/BaseLight.xaml
WalletWasabi.Fluent/Styles/Themes/BaseLight.xaml
<SolidColorBrush x:Key="TextForeground">#212121</SolidColorBrush>
<SolidColorBrush x:Key="AcrylicFallbackColor">#E2E2E2</SolidColorBrush>
<Color x:Key="RegionColor">#FFFAFAFA</Color>
+ <BoxShadow x:Key="BoxShadow">0 1 4 #D1D1D1</BoxShadow>
</Styles.Resources>
<StyleInclude Source="avares://Avalonia.Themes.Fluent/Accents/FluentLight.xaml"/>
</Styles>

MODIFY
WalletWasabi.Fluent/Views/MainView.axaml
WalletWasabi.Fluent/Views/MainView.axaml
<DockPanel>
<local:StatusBar DockPanel.Dock="Bottom" DataContext="{Binding StatusBar}" VerticalAlignment="Bottom" Height="32" TextBlock.FontSize="12" />
<Panel>
- <Border Background="{DynamicResource RegionColor}" BoxShadow="0 1 4 #131313" IsHitTestVisible="False" />
+ <Border Background="{DynamicResource RegionColor}" BoxShadow="{DynamicResource BoxShadow}" IsHitTestVisible="False" />
<DockPanel Margin="20 10 20 0">
<TextBlock Text="{Binding NavBar.SelectedItem.Title}" DockPanel.Dock="Top" FontSize="28" IsHitTestVisible="False" />
<rxui:RoutedViewHost Router="{Binding Router}">

add box shadow resource