Silverlight 4.0 allows you to define implicit styles that are applied automatically to all controls of the type specified in the Style TargetType property, you can create an implicit style by creating a Style without any key, for example here is an implicit style for the TextBlock control
This style will be applied to all the TextBlocks that don’t define an explicit style, excellent.. right?, unfortunately this will not work for the controls defined inside a Template, for example if you have a DataTemplate that contains a TextBlock, the TextBlock will not pick the implicit style, Microsoft explains the behaviour
“Templates are viewed as an encapsulation boundary when looking up an implicit style for an element which is not a subtype of Control.”
This explains why this doesn’t work for the TextBlocks (not a subtype of Control), the same settings will work for the Button control because it is a subtype of Control.
The solution is to use explicit styles or move the implicit style to be inside the template.
For more details please refer to the following links