What's causing styles to don't work with API level 16?

So I applied a custom style to my buttons, it works ok with API level 22, but when I test it with API level 16 all my custom styles are gone. What's happening here?. I am an android beginner so probably it's a common behavior but I couldn't find any answer.

Styles are split into 4 xml's representing button states: Pressed, focused, disabled and enabled. I grouped them in 1 xml called button.xml and applied it in styles.xml

button_pressed.xml (focused, disabled and enabled are similar)

<?xml version="1.0" encoding="utf-8"?>

<shape xmlns:android="http://ift.tt/nIICcg"
android:shape="rectangle">
<gradient
    android:angle="90"
    android:endColor="#004B8D"
    android:startColor="#0865B7" />
<size
    android:height="65dp"
    android:width="65dp"/>
<padding
    android:bottom="5dp"
    android:left="5dp"
    android:right="5dp"
    android:top="5dp" />
<stroke
    android:width="1dp"
    android:color="#FFFFFF" />
<corners android:radius="3dp" />
</shape>

button.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://ift.tt/nIICcg">

<item
    android:state_enabled="false"
    android:drawable="@drawable/button_disabled" />
<item
    android:state_pressed="true"
    android:state_enabled="true"
    android:drawable="@drawable/button_pressed" />
<item
    android:state_focused="true"
    android:state_enabled="true"
    android:drawable="@drawable/button_focused" />
<item
    android:state_enabled="true"
    android:drawable="@drawable/button_enabled" />

</selector>

styles.xml

<resources>

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
    <item name ="android:buttonStyle">@style/button</item>
</style>

<style name="button" parent="@android:style/Widget.Button">
    <item name="android:gravity">center_vertical|center_horizontal</item>
    <item name="android:textColor">#FFFFFFFF</item>
    <item name="android:shadowColor">#000000</item>
    <item name="android:shadowDx">0</item>
    <item name="android:shadowDy">-1</item>
    <item name="android:shadowRadius">0.4</item>
    <item name="android:textSize">20sp</item>
    <item name="android:textStyle">bold</item>
    <item name="android:background">@drawable/button</item>
    <item name="android:focusable">false</item>
    <item name="android:clickable">true</item>
    <item name="android:fontFamily">sans-serif-condensed</item>
</style>

via Chebli Mohamed