Windows Phone
Themes for Windows Phone
http://msdn.microsoft.com/en-us/library/ff769554(v=vs.92).aspx
User Experience Design Guidelines for Windows Phone
http://msdn.microsoft.com/en-us/library/hh202915(v=vs.92).aspx
Android
http://developer.android.com/design/index.html
iOS
iOS Human Interface Guidelines
http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/Introduction/Introduction.html