ghost-theme-sd.ai/assets/js/theme.js

// Toggle theme

const isOSLight = window.matchMedia("(prefers-color-scheme: light)").matches
const localTheme = window.localStorage && window.localStorage.getItem("lightTheme")
const themeToggle = document.querySelector(".theme-toggle")

function changeFavicon() {
  const src = document.documentElement.classList.contains("dark-theme") ? "/assets/images/favicon-dark.png" : "/assets/images/favicon.png";
  const link = document.createElement('link'),
    oldLink = document.getElementById('dynamic-favicon');
  link.id = 'dynamic-favicon';
  link.rel = 'shortcut icon';
  link.href = src;
  if (oldLink) {
    document.head.removeChild(oldLink);
  }
  document.head.appendChild(link);
}

function changeTheme(useLight) {
  document.documentElement.classList.remove('light-theme', 'dark-theme', 'has-light-text', 'has-dark-text')
  if (useLight) {
    document.documentElement.classList.add('light-theme', 'has-dark-text')
  } else {
    document.documentElement.classList.add('dark-theme', 'has-light-text')
  }

  if (window.localStorage) {
    window.localStorage.setItem(
        "lightTheme",
        useLight ? "light" : "dark")
  }

  changeFavicon()
}

themeToggle.addEventListener("click", () => {
  changeTheme(document.documentElement.classList.contains('dark-theme'))
});

if (localTheme === "light") {
  changeTheme(true)
} else {
  changeTheme(isOSLight)
}