I implemented a loading bar that shows while my script is running in the background in a Google Sheets, almost all the elements used in CSS actually work properly, the only thing that doesn´t work is an image thats suppose to be in the center of the loading bar.
I found out how to actually implement the CSS in Google Appscript here CSS with Google App Script. But whenever I use background-image: url(myimage.png); there is no image shown in the loading bar.
CSS
<!DOCTYPE html>
<html lang="es">
<head>
    <link rel="shortcut icon" href="../assets/images/ico/favicon.ico" />
    <meta name="sitedomain" content="www.sdfsd.com.mx" />
    <meta name="country" content="Am" />
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta name="description" content="">
    <meta name="author" content="">
    <link rel="stylesheet" type="text/css" href="loader.css"/>
</head>
    <body>  
    <style>
    .text-loader {
    font-family: Arial, sans-serif;
    font-size: 20px;
} 
.loader-align {
    width:100%; 
    height:100%; 
    margin: 0 auto;
}
.center {
    text-align: center;
}
.dextra-d {
    background-image: url(d_icon.png);
    background-repeat: no-repeat; 
    background-position: center;
}
@keyframes lds-double-ring {
  0% {
    -webkit-transform: rotate(0);
    transform: rotate(0);
  }
  100% {
    -webkit-transform: rotate(360deg);
    transform: rotate(360deg);
  }
}
@-webkit-keyframes lds-double-ring {
  0% {
    -webkit-transform: rotate(0);
    transform: rotate(0);
  }
  100% {
    -webkit-transform: rotate(360deg);
    transform: rotate(360deg);
  }
}
@keyframes lds-double-ring_reverse {
  0% {
    -webkit-transform: rotate(0);
    transform: rotate(0);
  }
  100% {
    -webkit-transform: rotate(-360deg);
    transform: rotate(-360deg);
  }
}
@-webkit-keyframes lds-double-ring_reverse {
  0% {
    -webkit-transform: rotate(0);
    transform: rotate(0);
  }
  100% {
    -webkit-transform: rotate(-360deg);
    transform: rotate(-360deg);
  }
}
.lds-double-ring {
  position: relative;
}
.lds-double-ring div {
  position: absolute;
  width: 160px;
  height: 160px;
  top: 20px;
  left: 20px;
  border-radius: 50%;
  border: 8px solid #000;
  border-color: #1d3f72 transparent #1d3f72 transparent;
  -webkit-animation: lds-double-ring 2.6s linear infinite;
  animation: lds-double-ring 2.6s linear infinite;
}
.lds-double-ring div:nth-child(2) {
  width: 140px;
  height: 140px;
  top: 30px;
  left: 30px;
  border-color: transparent #5699d2 transparent #5699d2;
  -webkit-animation: lds-double-ring_reverse 2.6s linear infinite;
  animation: lds-double-ring_reverse 2.6s linear infinite;
}
.lds-double-ring {
  width: 200px !important;
  height: 200px !important;
  -webkit-transform: translate(-100px, -100px) scale(1) translate(100px, 100px);
  transform: translate(-100px, -100px) scale(1) translate(100px, 100px);
}
    
    </style>
         
<div class="center">
    <div class="dextra-d">
    <div class="lds-double-ring loader-align">
        <div></div>
        <div></div>
    </div>
        </div>
 <span class="text-loader">Cargando entrevista...</span>   
</div>     
        </body>
</html><!DOCTYPE html>
<html>
<head>
<base target="_top">
</head>
<body>
<?!= include('LoadBar'); ?>
<script>
if (<?= close ?> == "close"){
google.script.host.close();
}
</script>
</body>
</html> 
     
     
    