script.js

const { body } = document;

function changeBackground(number) {
    //Check if background already showing
    let previousBackground;
    if (body.className) {
        previousBackground = body.className;
    }
    // Reset CSS class for body
    body.className = '';
    switch (number) {
        case '1':
            return (previousBackground === 'background-1' ? false : body.classList.add('background-1'));
        case '2':
            return (previousBackground === 'background-2' ? false : body.classList.add('background-2'));
        case '3':
            return (previousBackground === 'background-3' ? false : body.classList.add('background-3'));
        case '4':
            return (previousBackground === 'background-4' ? false : body.classList.add('background-4'));
        default:
            break;    
    }
}