Вы можете переопределить thumbs
класс css с дополнительным экземпляром:
<script type="text/javascript">
// once head is loaded add style element with class definition that will override .thumbs
var head = document.getElementsByTagName('head')[0]; // get head dom element
head.innerHTML += '<style> .thumbs{background:url("https://www.toptal.com/designers/subtlepatterns/patterns/leaves.png") no-repeat;}</style>'; // put additional style at the end of head (this will overide thumbs), you can put any url background here.
</script>
Рабочий пример ниже (https://jsbin.com/mehuhokopi/edit?html,output):
<html>
<head>
<style> .thumbs{background:url('/img/thumbs.jpg') no-repeat;}</style>
<style>
.test {
width: 100%;
height: 100%;
background-color: red;
}
</style>
<script type="text/javascript">
// once head is loaded add style element with class definition that will override .thumbs
var head = document.getElementsByTagName('head')[0]; // get head dom element
head.innerHTML += '<style> .thumbs{background:url("https://www.toptal.com/designers/subtlepatterns/patterns/leaves.png") no-repeat;}</style>'; // put additional style at the end of head (this will overide thumbs), you can put any url background here.
</script>
</head>
<body>
<div class="test thumbs">
</div>
</body>
</html>