You can override the style for the dropdown, and the dropdown item by using a Theme in your app that inherits from one of the Android themes, then override the
android:dropDownSpinnerStyle, or android:spinnerDropDownItemStyle, and even the android:dropDownListViewStyle attribute of the theme, pointing to your own custom style instead of the Android style that is defined in their theme. I created a fully customized spinner this way, with a transparent button AND dropdown. I even got rid of the dropdown list dividers, and set my own spacing for the dropdown items when I built the tablet app for Fandango (take a look at the sort movies spinner on the main page of the app).
Everything in Android is customizable, you just have to know where to look. 😉