<html>
<head>
<style>
:root { --one: 10px; --two: 20px; }
</style>
<body>
<script>
document.styleSheets[0].cssRules[0].style.setProperty("--one", "5px")
document.styleSheets[0].cssRules[0].style.removeProperty("--two")
document.styleSheets[0].cssRules[0].style.setProperty("--two", "30px")
document.styleSheets[0].cssRules[0].style.setProperty("--one", "50px")
document.write(document.styleSheets[0].cssRules[0].style.getPropertyValue("--one") + " " + document.styleSheets[0].cssRules[0].style.getPropertyValue("--two"))
</script>
</body>
</html.