Android for Beginners : Header Text Style Solution

activity_main.xml

<ScrollView 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"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin">

        <EditText
            android:id="@+id/name_field"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/name"
            android:inputType="text" />

        <TextView
            style="@style/HeaderTextStyle"
            android:text="@string/toppings" />

        <CheckBox
            android:id="@+id/whipped_cream_checkbox"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="24dp"
            android:text="@string/whipped_cream"
            android:textSize="16sp" />

        <CheckBox
            android:id="@+id/chocolate_checkbox"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="24dp"
            android:text="@string/chocolate"
            android:textSize="16sp" />

        <TextView
            style="@style/HeaderTextStyle"
            android:text="@string/quantity" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <Button
                android:layout_width="48dp"
                android:layout_height="48dp"
                android:onClick="decrement"
                android:text="-" />

            <TextView
                android:id="@+id/quantity_text_view"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:paddingLeft="8dp"
                android:paddingRight="8dp"
                android:text="@string/initial_quantity_value"
                android:textColor="@android:color/black"
                android:textSize="16sp" />

            <Button
                android:layout_width="48dp"
                android:layout_height="48dp"
                android:onClick="increment"
                android:text="+" />

        </LinearLayout>

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:onClick="submitOrder"
            android:text="@string/order" />

    </LinearLayout>
</ScrollView>

styles.xml

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
    </style>

    <!-- Style for header text in the order form -->
    <style name="HeaderTextStyle">
        <item name="android:layout_width">wrap_content</item>
        <item name="android:layout_height">48dp</item>
        <item name="android:gravity">center_vertical</item>
        <item name="android:textAllCaps">true</item>
        <item name="android:textSize">15sp</item>
    </style>
</resources>

what is the difference between styles and going through the android: function?

It's about whether you'll use again or not, if you use that ever again you may want to add that style to styles.xml file. 👍