MyTestStyles.css

body {

   background: url(https://www.mediawiki.org/wiki/MediaWiki#/media/File:Wikimedia_Hackathon_Prague_2019_-_Group_Photo_-_CLK_-_cropped.jpg) no-repeat center center fixed; 
   -webkit-background-size: cover;
   -moz-background-size: cover;
   -o-background-size: cover;
   background-size: cover;

}