Setting the theme for spinners when using an AppCompat theme.

themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="AppTheme" parent="@style/Theme.AppCompat.Light.NoActionBar">
        <item name="spinnerStyle">@style/Spinner</item>
        <item name="android:spinnerItemStyle">@style/Spinner.Item</item>
        <item name="spinnerDropDownItemStyle">@style/Spinner.Dropdown</item>
    </style>

    <!-- styling for the main widget -->
    <style name="Spinner" parent="@style/Widget.AppCompat.Spinner">
        <item name="android:background">@drawable/spinner_background</item>
    </style>

    <!-- styling for the text inside the main widget (includes padding, font, etc.) -->
    <style name="Spinner.Item" parent="@style/Widget.AppCompat.TextView.SpinnerItem">
        <item name="android:textColor">@color/text_color</item>
        <item name="android:textSize">16sp</item>
        <item name="android:fontFamily">@font/main_data_entry_font</item>
    </style>

    <!-- styling for drop down items -->
    <style name="Spinner.Dropdown" parent="@style/Widget.AppCompat.Spinner.DropDown">
        <item name="android:textColor">@color/text_color</item>
        <item name="android:textSize">16sp</item>
        <item name="android:fontFamily">@font/main_data_entry_font</item>
    </style>
</resources>