Windows 8 开发系列-如何修改系统样式

You have to set the Style and the Foreground in this way:  
<ProgressBar Foreground="Yellow" IsIndeterminate="True" Style="{StaticResource CustomProgressBarStyle}"/>  

 

You can also find default themes styles in the folder "C:\Program Files (x86)\Windows Kits\8.0\Include\winrt\xaml\design".