styles.xml

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="MainTheme.Base">
       <!-- As of Xamarin.Forms 4.6 the theme has moved into the Forms binary -->
        <!-- If you want to override anything you can do that here. -->
        <!-- Underneath are a couple of entries to get you started. -->

        <!-- Set theme colors from https://aka.ms/material-colors -->
        <!-- colorPrimary is used for the default action bar background -->
        <item name="colorPrimary">#2196F3</item>
        <!-- colorPrimaryDark is used for the status bar -->
        <item name="colorPrimaryDark">#1976D2</item>
        <!-- colorAccent is used as the default value for colorControlActivated
             which is used to tint widgets -->
        <item name="colorAccent">#3498db</item>
    </style>

</resources>