Fix theme switching failure caused because missing values

[scopewu]

The current code is able to switch themes according to the system state. The problem is that when the site is loaded and the system state changes, the theme switching failure caused because missing values. There are two fixes:
A. Determine the missing values when the state changes:

window.matchMedia("(prefers-color-scheme: dark)")
  .addEventListener("change", event => {
    if (localStorage.theme === "system" || !localStorage.theme) {
      toggleTheme(event.matches);
    }
  }
);

B. Set missing values when loading:

function preloadTheme() {
  const userTheme = localStorage.theme;

  if (!userTheme) {
    localStorage.setItem("theme", "system");
  }

  if (userTheme === "light" || userTheme === "dark") {
    toggleTheme(userTheme === "dark");
  } else {
    toggleTheme(window.matchMedia("(prefers-color-scheme: dark)").matches);
  }
}

Here I have chosen the first way(A).

Thanks for your work.