Custom Theme That Overrides Default Theme WP7

Is it possible to create a custom theme and have it used as the default theme?

Every example I can find anywhere says you can create custom themes by copying the ThemeResources.xaml and System.Windows.xaml files and including them as merged dictionaries in your app.

http://windowsphonegeek.com/articles/Creating-WP7-Custom-Theme-ndash-Basic-Theme-Implementation Overriding themes in Windows Phone 7

Then I've read some more that you need to include the brushes inside the styles file, so in CustomThemeStyles.xaml I have this.

http://www.windowsphonegeek.com/articles/Creating-WP7-Custom-Theme---Complex-Theme

It doesn't work... so I download the sample app and sure enough, every page that wants to have some color changed, like the background color, will set it on it's outer most component.

...

Is it possible to include custom themes that change the style/brushes/colors/etc of all the defaults without having to explicitly set them everywhere?