script that implements a window api to control the user's choice of theme with localstorage support and without the flash bug

darkmode.js

//window.__theme: variable with the current theme: "dark" or "light"
//window.__setPreferredTheme: function that receives the new theme to be applied
//window.__onThemeChange: function that will be called every time the theme changes

(function() {
    window.__onThemeChange = function() {};
    function setTheme(newTheme) {
        window.__theme = newTheme;
        preferredTheme = newTheme;
        document.body.className = newTheme;
        window.__onThemeChange(newTheme);
    }
    var preferredTheme;
    try {
        preferredTheme = localStorage.getItem("theme");
    } catch (err) {}
    window.__setPreferredTheme = function(newTheme) {
        setTheme(newTheme);
        try {
            localStorage.setItem("theme", newTheme);
        } catch (err) {}
    };
    setTheme(preferredTheme || "dark");
})();