Template:Sandbox/styles.css

.test-auto {
	--color-l: green;
	--color-d: red;

	color: var(--color-l);
}

html.skin-theme-clientpref-night .test-auto {
	color: var(--color-d);
}

@media (prefers-color-scheme: dark) {
	html.skin-theme-clientpref-os .test-auto {
		color: var(--color-d);
	}
}