//styles.xml  file

<?xml version="1.0" encoding="UTF-8" ?>
<resources>
    <!-- Inherit from the light Material Theme -->
    <style name="MyCustomTheme" parent="android:Theme.Material.Light">
        <!-- Override the app bar color -->
        <item name="android:colorPrimary">@color/green</item>
        <!-- Override the color of UI controls -->
        <item name="android:colorAccent">@color/white</item>
           <item name="android:statusBarColor">@color/orange</item>
    </style>
</resources>


// in Mainfest file
<application android:label="MyApp"
    android:theme="@style/MyCustomTheme">
</application>

// in activity
[Activity(Label = "MyActivity", Theme = "@style/MyCustomTheme")]


//link
//https://developer.xamarin.com/guides/android/user_interface/material_theme/