bitwarden-browser/apps/web/src/theme.js

// Set theme on page load
// This is done outside the Angular app to avoid a flash of unthemed content before it loads
// The defaultTheme is also set in the html itself to make sure that some theming is always applied
(function () {
  const defaultTheme = "light";
  const htmlEl = document.documentElement;
  let theme = defaultTheme;

  const globalState = window.localStorage.getItem("global");
  if (globalState != null) {
    const globalStateJson = JSON.parse(globalState);
    if (globalStateJson != null && globalStateJson.theme != null) {
      if (globalStateJson.theme.indexOf("system") > -1) {
        theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
      } else if (globalStateJson.theme.indexOf("dark") > -1) {
        theme = "dark";
      }
    }
    if (!htmlEl.classList.contains("theme_" + theme)) {
      htmlEl.classList.remove("theme_" + defaultTheme);
      htmlEl.classList.add("theme_" + theme);
    }
  }
})();