Overriding implict style breaks ThemeResource references?

Question

In a Windows 8.1 store app, when I define an implicit style, say for a TextBox, the light/dark theme changes I make to built-in ThemeResources seem to be ignored.
For instance,
I've set up a ThemeResource file for the Default (Dark) theme, and set the color of TextBoxBackgroundThemeBrush to Lime. 
I then copied the default TextBox template to two different resource dictionary files and made a change to the border color and width (for visual cues).  The files are identical except for defining an x:Key in the "explicit" theme file.
The three XAML files (my theme resource dictionary and the two style files are added to App.xaml as MergedDictionaries.
I added two textboxes to a vanilla page - one has the explicit style applied, one relies on the implicit style... the result is below:
Note that the Implicit Style has lost the Lime background, even though it refers to {ThemeResource TextBoxBackgroundThemeBrush} just like the explicit style.

Answer 1

Hi Jim,
Thanks for your repro, I can see the exactly behavior as you describe in your question.
However I think the behavior could be explained by the Lookup behavior for XAML resource references, you may find more information from here:
ResourceDictionary and XAML resource references.
If a resource isn't found in the initial scope, the scope expands. The lookup behavior continues on throughout the locations and scopes that a XAML resource could possibly be defined by an app or by the system.
When the TextBox is apply implicit style, the initial scope is system resource dictionary and there is the default setting for TextBoxBackgroundThemeBrush, then it take the default color. Again another small test, if we modify the implicit style by changing
the resource key name, also we change the name in DarkTheme.xaml
ImplicitTextBoxStyle.xaml:
<Style TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}" />
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}" />
<Setter Property="Foreground" Value="{ThemeResource TextBoxForegroundThemeBrush}" />
<Setter Property="Background" Value="{ThemeResource TextBoxBackgroundThemeBrush1}" />
DarkTheme.xaml:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<ResourceDictionary.ThemeDictionaries>
<ResourceDictionary x:Key="Default">
<SolidColorBrush x:Key="TextBoxBackgroundThemeBrush1" Color="Lime" />
</ResourceDictionary>
</ResourceDictionary.ThemeDictionaries>
</ResourceDictionary>
The error will be thrown while we tried to debug it, looks like the Resource cannot be found. Probably the expend is outward expansion but not downward expansion.
When the TextBox is apply explicit style, the initial scope is ResourceDictionary, and system will look into all the ResourceDictionaries to find the correct key, then it stop, Lime color will be displayed.
Note  It is a common practice to define all the immediate resources at the root level of a page, both to take advantage of this resource-lookup behavior and also as a convention of XAML markup style.
--James
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