The code below enables web-apps to be working in dark/light mode. It needs to be added to the app settings custom code header.
<script src="https://cdn.jsdelivr.net/npm/darkmode-js@1.5.7/lib/darkmode-js.min.js"></script>
<script>
function addDarkmodeWidget() {
new Darkmode().showWidget();
}
window.addEventListener('load', addDarkmodeWidget);
</script>