theme.js

(() => {
  const root = document.documentElement;
  const toggle = document.querySelector(".theme-toggle");
  const icon = toggle && toggle.querySelector(".theme-icon");
  const label = toggle && toggle.querySelector(".theme-label");
  const systemTheme = window.matchMedia("(prefers-color-scheme: dark)");

  if (!toggle || !icon || !label) return;

  const updateToggle = () => {
    const isDark = root.dataset.theme === "dark";
    icon.textContent = isDark ? "☀️" : "🌙";
    label.textContent = isDark ? "日间模式" : "夜间模式";
    toggle.setAttribute("aria-label", isDark ? "切换到日间模式" : "切换到夜间模式");
    toggle.setAttribute("aria-pressed", String(isDark));
  };

  toggle.addEventListener("click", () => {
    const nextTheme = root.dataset.theme === "dark" ? "light" : "dark";
    root.dataset.theme = nextTheme;
    try {
      localStorage.setItem("theme", nextTheme);
    } catch (error) {
      // 本地存储不可用时，切换仍对当前页面有效。
    }
    updateToggle();
  });

  const followSystemTheme = (event) => {
    try {
      if (localStorage.getItem("theme")) return;
    } catch (error) {
      // 无法读取本地存储时继续跟随系统主题。
    }
    root.dataset.theme = event.matches ? "dark" : "light";
    updateToggle();
  };

  if (systemTheme.addEventListener) {
    systemTheme.addEventListener("change", followSystemTheme);
  } else {
    systemTheme.addListener(followSystemTheme);
  }

  updateToggle();
})();