You can directly give the Key(NavigationViewExpandedPaneBackGround) to your AcrylicBrush. So it will change your navigation view background.

  <Page.Resources>
      <ResourceDictionary>
            <ResourceDictionary.ThemeDictionaries>
                <ResourceDictionary x:Key="Light">
                    <AcrylicBrush x:Key="NavigationViewExpandedPaneBackground" BackgroundSource="HostBackdrop" TintColor="{ThemeResource SystemAccentColorDark1}" FallbackColor="{ThemeResource SystemAccentColorDark1}" TintOpacity="0.80"/>   
                </ResourceDictionary>
                <ResourceDictionary x:Key="Dark">
                    <AcrylicBrush x:Key="NavigationViewExpandedPaneBackground" BackgroundSource="HostBackdrop" TintColor="{ThemeResource SystemAltHighColor}" FallbackColor="#333333" TintOpacity="0.50"/>
                </ResourceDictionary>
            </ResourceDictionary.ThemeDictionaries>
        </ResourceDictionary>
</Page.Resources>

Hope this solves your problem.