devthread.ru/src/components/Header/ThemeSwitcher/ThemeSwitcher.js

import React from 'react';

import MoonIcon from "../../../images/moon-icon.svg"
import SunIcon from "../../../images/sun-icon.svg"

const ThemeSwitcher = ({ ready, isDark, setIsDark }) => {
  // const [ready, setReady] = React.useState(false)
  // const [isDark, setIsDark] = React.useState(false)
  
  // React.useEffect(() => {
  //   setIsDark(window.__isDarkTheme)
  //   setReady(true)
  // }, [setIsDark, setReady])

  return (
    <div className={ready ? 'header-nav__theme ready' : 'header-nav__theme'}>
      {isDark ? (
        <SunIcon
          onClick={() => {
            window.__setTheme("light")
            setIsDark(!isDark)
          }}
        />
      ) : (
        <MoonIcon
          onClick={() => {
            window.__setTheme("dark")
            setIsDark(!isDark)
          }}
        />
      )}
    </div>
  )
}

export default ThemeSwitcher