html {  
       background: url(2015.jpg) no-repeat center center fixed; 
       -webkit-background-size: cover;
       -moz-background-size: cover;
       -o-background-size: cover;
       background-size: cover;
       }

#x {
     margin: 25% 25%;
     padding: 5px;
     font-family:  Arial, sans-serif;
     font-size: 30px;
     color: #111;
     opacity: .85;
     background: #ccc; 
     text-align: center; 
     }
#z { 
     font-size: 16px;
     }
#a { 
     font-size: 12px;
     }
a {
    color: #333;
    }