window.onload = function() {
//DsPdfViewer.LicenseKey = "***key***";
const viewer = new DsPdfViewer("#viewer", { workerSrc: "/document-solutions/javascript-pdf-viewer/demos/product-bundles/build/dspdfviewer.worker.js" });
viewer.addDefaultPanels();
viewer.open("/document-solutions/javascript-pdf-viewer/demos/product-bundles/assets/pdf/wetlands.pdf");
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<title>Override CSS Styles</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link rel="stylesheet" href="./src/styles.css">
<script src="/document-solutions/javascript-pdf-viewer/demos/product-bundles/build/dspdfviewer.js"></script>
<script src="/document-solutions/javascript-pdf-viewer/demos/resource/js/init.js"></script>
<script src="./src/app.js"></script>
</head>
<body>
<div id="viewer"></div>
</body>
</html>
#viewer { height: 100%; }
body .gc-menu__btn-container { background-color: #999999 !important; }
body .gc-btn--accent { background-color: #000000 !important; }
body .gc-btn--accent:not([disabled]):not(.gc-btn--disabled):hover { background-color: #000000; }
body .gc-viewer-host .gc-viewer .gcv-menu .gc-menu__panel-toggle--active .gc-btn { background-color: #999999; }
body .gc-btn[disabled] { opacity: 0.7; }
body .gc-accent-color { color: #7914a2 !important; }
body .gc-menu__panel-header { color: #999999; }
body .gc-pdfthumbnails-outer .gc-pdfthumbnails-inner .thumbnail .label { color: #999999; }
body .gc-pdfthumbnails-outer .gc-pdfthumbnails-inner .thumbnail.selected .selectdecor { background-color: #000000; }
body .gc-pdfthumbnails-outer .gc-pdfthumbnails-inner .thumbnail:hover .selectdecor { background-color: #7914a2; }
body .gcv-page-input__text { color: #7914a2; }
