misskey/src/client/app/common/scripts/theme.ts

import * as tinycolor from 'tinycolor2';

export default function(theme: { [key: string]: string }) {
	const props = compile(theme);

	Object.entries(props).forEach(([k, v]) => {
		if (k == 'meta') return;
		document.documentElement.style.setProperty(`--${k}`, v.toString());
	});

	localStorage.setItem('theme', JSON.stringify(props));
}

function compile(theme: { [key: string]: string }): { [key: string]: string } {
	function getColor(code: string): tinycolor.Instance {
		// ref
		if (code[0] == '@') {
			return getColor(theme[code.substr(1)]);
		}

		return tinycolor(code);
	}

	const props = {};

	Object.entries(theme).forEach(([k, v]) => {
		if (k == 'meta') return;
		const c = getColor(v);
		props[k] = genValue(c);
		props[`${k}-r`] = c.toRgb().r;
		props[`${k}-g`] = c.toRgb().g;
		props[`${k}-b`] = c.toRgb().b;
		props[`${k}-a`] = c.toRgb().a;
	});

	const primary = getColor(props['primary']);

	for (let i = 1; i < 10; i++) {
		const color = primary.clone().setAlpha(i / 10);
		props['primaryAlpha0' + i] = genValue(color);
	}

	for (let i = 1; i < 100; i++) {
		const color = primary.clone().lighten(i);
		props['primaryLighten' + i] = genValue(color);
	}

	for (let i = 1; i < 100; i++) {
		const color = primary.clone().darken(i);
		props['primaryDarken' + i] = genValue(color);
	}

	return props;
}

function genValue(c: tinycolor.Instance): string {
	return c.toRgbString();
}