[рони]

vitalso78,
так сделайте

document.addEventListener("DOMContentLoaded", function() {
            const styleElement = document.querySelector("#theme-css-file");
            const button = document.querySelector("#SuperButton");
            let CssFilePath = localStorage.getItem("CssFilePath");
            if (CssFilePath) {
                if (CssFilePath.indexOf("dark.css") !==-1) button.classList.add("switch-on");
                else button.classList.remove("switch-on");
                styleElement.href = CssFilePath;
            }
            button.addEventListener("click", function() {
                let href = "https://delivery.foodsonic.ru/light.css";
                button.classList.toggle("switch-on");
                if (button.classList.contains("switch-on")) href = "https://delivery.foodsonic.ru/dark.css";
                styleElement.href = href;
                localStorage.setItem("CssFilePath", href);
            })
        });