Signal-Android/app/src/main/res/drawable/add_media.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="rectangle">
            <size android:width="36dp" android:height="36dp" />
            <stroke android:color="@color/core_white" android:width="1.5dp" />
            <corners android:radius="10dp" />
            <solid android:color="@color/transparent_black_60" />
            <padding android:bottom="8dp" android:left="8dp" android:right="8dp" android:top="8dp" />
        </shape>
    </item>
    <item android:drawable="@drawable/ic_plus_20" />
</layer-list>