<html>
<style>
body {
    margin: -100px 0 0 0;
}

.inner {
    background-color: var(--backgroundColor);
    height: 100px;
    width: 100px;
}
</style>
<body>
<asdfasdf></asdfasdf>
<div style="--backgroundColor: red;">
    <div class="inner" id="a"></div>
</div>
<div style="--backgroundColor: green;">
    <div class="inner" id="b"></div>
</div>
</body>
</html>