Why is the floating action button displayed incorrectly in the xml file?

Question

Android

Added an Action Floating Button to the xml markup, but the button doesn't look like many of the examples. What could be the reason?
How it should be:

Like mine:

Icon :

xml file markup:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ListView
        android:id="@+id/listItems"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

    </ListView>


    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
       android:src="@drawable/plus"

        android:layout_margin="16dp" />

</RelativeLayout>

Answer 1

Here you can see how I connected

<android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_marginRight="16dp"
        android:layout_marginBottom="40dp"
        app:borderWidth="0dp"
        app:elevation="6dp"
        app:backgroundTint="@color/IDЦвета"
        app:srcCompat="@drawable/IDТвоейИконки"
        android:layout_marginEnd="16dp" />

where IDColors - the background for the button
YourIDIcons - the path to the icon