css/css-values/urls/support/relative-urls.css

:root {
    --image-path-stylesheet: url("images/test.png");
}

#stylesheet-relative-image {
    background-image: url(images/test.png);
}

#stylesheet-relative-variable-image {
    background-image: var(--image-path-stylesheet);
}

#stylesheet-relative-document-variable-image {
    background-image: var(--image-path-document);
}