script.js

var signup_link = document.querySelector('.a1');
var login_link = document.querySelector('.a2');

var signup_form = document.querySelector('.signup-form');
var login_form = document.querySelector('.login-form');

login_link.addEventListener('click', signup_hide);
signup_link.addEventListener('click', login_hide);

function signup_hide(){
	signup_form.style.display = "none";
	login_form.style.display = "block";
	signup_link.style.background = "#34495e";
	login_link.style.background = "#2981bc";
}
function login_hide(){
	signup_form.style.display = "block";
	login_form.style.display = "none";
	signup_link.style.background = "#2981bc";
	login_link.style.background = "#34495e";
}