Problem styling a Button, cannot assign properties.

Hello, I'm trying to style buttons in my app. I've followed the instructions here:
https://developer.xamarin.com/recipes/android/resources/general/style_a_button/
It works fine if I do exactly as it's explained, in a new project, and put the buttons in a layout axml file, but when I port it to my real app and put the script:

    <Button
        android:background="@drawable/red_button"
        android:id="@+id/button1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Styled Button"
        style="@style/button_text" />

in a regular XAML page it complains that "Cannot assign property "background": Property does not exists, or is not assignable, or mismatching type between value and property", for all the properties.

Why is that?

Thanks.