Here is a list of the general issues that has been reported to us and also an information on how to resolve them:
If you use Windows8 or Windows8Touch theme by defining the ApplicationTheme with StyleManager or by merging dictionaries in App.xaml, and you define your own implicit style (style without x:Key, defined in the resources) in which you are using Binding for the FontSize property, it will not be applied and the FontSize will be 0. This is a bug in Silverlight Framework. There are two solutions of the problem:
Define a key for your implicit style and set the controls to use this static resource.
Do not define the FontSize in your implicit style - set it directly on the controls.