material-dialogs/library/src/main/res/drawable/md_btn_selector_dark.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" android:exitFadeDuration="@android:integer/config_shortAnimTime">

    <item android:drawable="@drawable/md_btn_selected_dark" android:state_pressed="true" />
    <item android:drawable="@drawable/md_transparent" />

</selector>