HTMLify

const body = document.body;
const slides = document.querySelectorAll(".slide");
const leftButton = document.getElementById("left");
const rightButton = document.getElementById("right");

let activeSlide = 0;

const setBackground = () => {
  body.style.backgroundImage = slides[activeSlide].style.backgroundImage;
};

const setActiveSlide = () => {
  slides.forEach((slide) => slide.classList.remove("active"));
  slides[activeSlide].classList.add("active");
};

rightButton.addEventListener("click", () => {
  activeSlide++;
  if (activeSlide > slides.length - 1) activeSlide = 0;
  setBackground();
  setActiveSlide();
});

leftButton.addEventListener("click", () => {
  activeSlide--;
  if (activeSlide < 0) activeSlide = slides.length - 1;
  setBackground();
  setActiveSlide();
});

setBackground();