Add theme to spinner
<Spinner style="@style/SpinnerTheme"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
Add spinnerTheme to styles.xml
<style name="SpinnerTheme" parent="android:Widget.Spinner">
<item name="android:background">@drawable/spinner_background</item>
</style>
Add New -> "Vector Asset" to drawable with e.g. "ic_keyboard_arrow_down_24dp"
Add spinner_background.xml to drawable
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item>
<layer-list>
<item android:drawable="@drawable/ic_keyboard_arrow_down_24dp" android:gravity="center_vertical|right" android:right="5dp"/>
</layer-list>
</item>
</selector>