intent/static/js/theme.js

// Three-way theme choice (system, light, dark), stored per browser.
// Loaded in <head> so the saved theme applies before first paint.
(function () {
  var KEY = "intent-theme";
  var root = document.documentElement;

  function read() {
    try {
      return localStorage.getItem(KEY);
    } catch (e) {
      return null;
    }
  }

  function write(value) {
    try {
      if (value === "system") localStorage.removeItem(KEY);
      else localStorage.setItem(KEY, value);
    } catch (e) {}
  }

  function apply(value) {
    if (value === "light" || value === "dark") root.dataset.theme = value;
    else delete root.dataset.theme;
  }

  apply(read());

  document.addEventListener("DOMContentLoaded", function () {
    var form = document.getElementById("theme-form");
    var select = document.getElementById("theme-select");
    if (!form || !select) return;
    select.value = root.dataset.theme || "system";
    select.addEventListener("change", function () {
      apply(select.value);
      write(select.value);
    });
    form.hidden = false;
  });
})();