Use the below css Code to add multiple background image with gradient color to your website. You can change the color and image as you wish.
<style> body{background: url(http://www.creativetweets.com/wp-content/uploads/2018/04/background-neural-net.png) fixed,url(http://www.creativetweets.com/wp-content/uploads/2018/04/background-neural-bg-blur.png) fixed,linear-gradient(to right,#1067c4 10%,#1ebad2 100%);} </style>