dark mode palette custom properties

style.css

@media (prefers-color-scheme: dark) {
	body {
		--wp--custom--color--body: #fff;
	}
}

theme.json

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"settings": {
		"custom": {
			"body": "#000"
		},
		"color": {
			"palette": [
				{
					"name": "Body",
					"slug": "body",
					"color": "var(--wp--custom--color--body)"
				}
			]
		}
	}
}