hugo-academic/assets/js/load-theme.js

function getThemeMode() {
  return parseInt(localStorage.getItem('dark_mode') || 2);
}

let currentThemeMode = getThemeMode();
let isDarkTheme;
switch (currentThemeMode) {
  case 0:
    isDarkTheme = false;
    break;
  case 1:
    isDarkTheme = true;
    break;
  default:
    if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
      // The visitor prefers dark themes and switching to the dark variation is allowed by admin.
      isDarkTheme = true;
    } else if (window.matchMedia('(prefers-color-scheme: light)').matches) {
      // The visitor prefers light themes and switching to the dark variation is allowed by admin.
      isDarkTheme = false;
    } else {
      // Use the site's default theme variation based on `light` in the theme file.
      isDarkTheme = isSiteThemeDark;
    }
    break;
}
if (isDarkTheme) {
  document.body.classList.add("dark");
} else {
  document.body.classList.remove("dark");
}