hamann-ausgabe-core/HaWeb/wwwroot/js/theme.js

// Functions for switching theme
const go_to_dark = function () {
    localStorage.setItem("theme", "ha-toggledark");
    document.documentElement.classList.add("dark");
};

const go_to_bright = function () {
    document.documentElement.classList.remove("dark");
    localStorage.setItem("theme", "ha-togglebright");
};

// Functions for reading theme settings
const get_theme_settings = function (standard) {
    var theme = localStorage.getItem("theme");
    if (theme === null) theme = standard;
    let toggleSwitch = document.getElementById(theme).click();
};

if (
    document.getElementById("ha-togglebright") !== null &&
    this.document.getElementById("ha-toggledark") !== null
) {
    document
        .getElementById("ha-togglebright")
        .addEventListener("click", go_to_bright);
    document
        .getElementById("ha-toggledark")
        .addEventListener("click", go_to_dark);
}
get_theme_settings("ha-togglebright");