script.js

let onSwitch = document.getElementById("on");
let offSwitch = document.getElementById("off");
let onLabel = document.querySelector("label[for='on']");
let offLabel = document.querySelector("label[for='off']");
let container = document.getElementById("container");

let onClicked = () => {
  container.style.boxShadow = "inset -0.9em 0 0.9em #494949";
  offLabel.style.color = "#8e8e8e";
  onLabel.style.color = "#98f195";
};

let offClicked = () => {
  container.style.boxShadow = "inset 0.9em 0 0.9em #171717";
  onLabel.style.color = "#8e8e8e";
  offLabel.style.color = "#f1959f";
};

window.addEventListener("load", () => {
  if (onSwitch.checked) {
    onClicked();
  } else {
    offClicked();
  }
});

onSwitch.addEventListener("click", onClicked);
offSwitch.addEventListener("click", offClicked);