How to put line dividing items into a Spinner?

I'm trying to put a line dividing the items I have into Spinner but I can not seem to get it.

spinner_item.xml

<?xml version="1.0" encoding="utf-8"?>
    <TextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@android:id/text1"
        style="@style/spinnerItemStyle"
        android:maxLines="1"
        android:textColor="@color/grey_default"
        android:textStyle="bold"
        android:textSize="12dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ellipsize="marquee" />

spinner_dropbox.xml

<?xml version="1.0" encoding="utf-8"?>
<CheckedTextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/text1"
    style="@style/spinnerDropDownItemStyle"
    android:maxLines="1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:ellipsize="marquee" />

styles.xml

<style name="spinnerItemStyle" parent="android:Widget.TextView.SpinnerItem"/>

<style name="spinnerDropDownItemStyle" parent="Widget.AppCompat.ListView.DropDown">
    <item name="android:dividerHeight">2dip</item>
    <item name="android:divider">#808080</item>
</style>

MainActivity (...)

 //adapter creation categorias
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.spinner_item, list_categorias);
        adapter.setDropDownViewResource(R.layout.spinner_dropbox);
        categoria.setAdapter(adapter);

What I'm getting is this:

Can anyone help? Why not draw the line between the various items?