在运行时更改资源字典

Question

我在wpf中使用Material design。我在运行时更改主题（亮/暗）。

<materialDesign:DialogHost Identifier="RootDialog">
  <materialDesign:DialogHost.Resources>
    <Style TargetType="materialDesign:Card" BasedOn="{StaticResource {x:Type materialDesign:Card}}">
        <Style.Resources>
            <ResourceDictionary>
                <ResourceDictionary.MergedDictionaries>
                    <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesignTheme.dark.xaml" />
                </ResourceDictionary.MergedDictionaries>
            </ResourceDictionary>
        </Style.Resources>
        <Setter Property="Background" Value="{DynamicResource MaterialDesignCardBackground}" />
    </Style>
  </materialDesign:DialogHost.Resources>
  <Grid>
      <!--some code-->
  </Grid>
</materialDesign:DialogHost>

我想在运行时将资源字典的来源更改为

pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesignTheme.light.xaml

我该怎么做？