How to get back to the default foregound themed brush?

Hello,

TextBox controls in our themes currently have this default Foreground:

<Setter Property="Foreground" Value="{DynamicResource {x:Static themes:AssetResourceKeys.ContainerForegroundLowNormalBrushKey}}" />

You'd need to do something like this to reestablish the DynamicResource via code:

textBox.SetResourceReference(TextBox.ForegroundProperty, AssetResourceKeys.ContainerForegroundLowNormalBrushKey);

Hello,

I meant that when it changes to theme Metro.Dark, what is the foregound brush for the textbox? And vice-versa to Metro.Light.

Yes, this way:
Brush b = Application.Current.FindResource(AssetResourceKeys.ControlBackgroundNormalBrushKey) as Brush;

Hello,

If you get our latest version (released Friday) and run the Theme Designer app, you can load up the themes you are interested in from the Presets menu.  Then open the Resource Browser tab and filter the resource list down by ContainerForegroundLowNormalBrushKey.  If you right-click the item in the list, there now is a Copy Color Value menu item you can use to get the color in the currently-loaded theme.  If you only care about the color value used, that is an alternate way to get the brush's color value.