Read original post at Windows Phone 7 Nuggets
When Windows Phone first came out, setting custom themes was easy. Take a copy of ThemeResources from ‘c:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.1\Design\’ and swap the colours out with the theme colours you wanted. This changed when the 7.1 SDK came out that meant the default colours weren’t overridden. So what then? Retemplate all the controls to use the colours you want? No.