style pseudo element, style before, style after
<style>
div {
width: 100px;
height: 100px;
position: relative;
border: 1px solid black;
}
div:after {
background-color: var(--my-color-var);
content: '';
position: absolute;
top: 0;
bottom: 0;
right: 0;
left: 0;
}
</style>
<div style="--my-color-var: orange;"></div>