session-android/res/drawable-v21/conversation_list_item_background_dark.xml

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
        android:color="@color/textsecure_primary">
    <item android:id="@android:id/mask" android:drawable="@android:color/black" />
    <item>
        <selector>
            <item android:drawable="@color/textsecure_primary_alpha33" android:state_selected="true" />
        </selector>
    </item>
</ripple>