Android Studio - Night/Day

activity_main_nd2.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Seleziona l'aspetto"
        android:textColor="@color/textColor"/>

    <RadioGroup
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:checkedButton="@id/rB_day">

        <RadioButton
            android:id="@+id/rB_day"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Chiaro"
            android:textColor="@color/textColor" />

        <RadioButton
            android:id="@+id/rB_night"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Scuro"
            android:textColor="@color/textColor" />

        <RadioButton
            android:id="@+id/rB_default"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Predefinito da sistema"
            android:textColor="@color/textColor" />
    </RadioGroup>

</LinearLayout>