CodePen Embed - Responsive Contact Page: dark mode + form validation

<!-- Responsive Contact Page with Dark Mode and Form Validation (vanilla JS).

*Designed & built for desktop and tablets with viewport width >= 720px and in landscape orientation.  -->

<div class="contact-container">
  <div class="left-col">
    <img class="logo" src="https://www.indonesia.travel/content/dam/indtravelrevamp/en/logo.png"/>
  </div>
  <div class="right-col">
    <div class="theme-switch-wrapper">
    <label class="theme-switch" for="checkbox">
        <input type="checkbox" id="checkbox" />
        <div class="slider round"></div>
  </label>
  <div class="description">Dark Mode</div>
</div>
    
    <h1>Contact us</h1>
    <p>Planning to visit Indonesia soon? Get insider tips on where to go, things to do and find best deals for your next adventure.</p>
    
    <form id="contact-form" method="post">
      <label for="name">Full name</label>
  <input type="text" id="name" name="name" placeholder="Your Full Name" required>
      <label for="email">Email Address</label>
  <input type="email" id="email" name="email" placeholder="Your Email Address" required>
      <label for="message">Message</label>
  <textarea rows="6" placeholder="Your Message" id="message" name="message" required></textarea>
      <!--<a href="javascript:void(0)">--><button type="submit" id="submit" name="submit">Send</button><!--</a>-->
  
</form>
<div id="error"></div>
<div id="success-msg"></div>
  </div>
</div>

<!-- Image credit: Oliver Sjöström https://www.pexels.com/photo/body-of-water-near-green-mountain-931018/  -->

:root {
  --primary-color: #010712;
  --secondary-color: #818386;
  --bg-color: #FCFDFD;
  --button-color: #3B3636;
  --h1-color: #3F444C;
}

[data-theme="dark"] {
  --primary-color: #FCFDFD;
  --secondary-color: #818386;
  --bg-color: #010712;
  --button-color: #818386;
  --h1-color: #FCFDFD;
}

* {
  margin: 0;
  box-sizing: border-box;
  transition: all 0.3s ease-in-out;
}

.contact-container {
  display: flex;
  width: 100vw;
  height: 100vh;
  background: var(--bg-color);
}

.left-col {
  width: 45vw;
  height: 100%;
  background-image: url("https://images.pexels.com/photos/931018/pexels-photo-931018.jpeg?auto=compress&cs=tinysrgb&dpr=2&w=500");
  background-size: cover;
  background-repeat: no-repeat;
}

.logo {
  width: 10rem;
  padding: 1.5rem;
}

.right-col {
  background: var(--bg-color);
  width: 50vw;
  height: 100vh;
  padding: 5rem 3.5rem;
}

h1, label, button, .description {
  font-family: 'Jost', sans-serif;
  font-weight: 400;
  letter-spacing: 0.1rem;
}

h1 {
  color:var(--h1-color);
  text-transform: uppercase;
  font-size: 2.5rem;
  letter-spacing: 0.5rem;
  font-weight: 300;
}

p {
  color: var(--secondary-color);
  font-size: 0.9rem;
  letter-spacing: 0.01rem;
  width: 40vw;
  margin: 0.25rem 0;
}

label, .description {
  color: var(--secondary-color);
  text-transform: uppercase;
  font-size: 0.625rem;
}

form {
  width: 31.25rem;
  position: relative;
  margin-top: 2rem;
  padding: 1rem 0;
}

input, textarea, label {
  width: 40vw;
  display: block;
}

p, placeholder, input, textarea {
  font-family: 'Helvetica Neue', sans-serif;
}

input::placeholder, textarea::placeholder {
  color: var(--primary-color);
}

input, textarea {
  color: var(--primary-color);
  font-weight: 500;
  background: var(--bg-color);
  border: none;
  border-bottom: 1px solid var(--secondary-color);
  padding: 0.5rem 0;
  margin-bottom: 1rem;
  outline: none;
}

textarea {
  resize: none;
}

button {
  text-transform: uppercase;
  font-weight: 300;
  background: var(--button-color);
  color: var(--bg-color);
  width: 10rem;
  height: 2.25rem;
  border: none;
  border-radius: 2px;
  outline: none;
  cursor: pointer;
}

input:hover, textarea:hover, button:hover {
  opacity: 0.5;
}

button:active {
  opacity: 0.8;
}

/* Toggle Switch */

.theme-switch-wrapper {
  display: flex;
  align-items: center;
  text-align: center;
  width: 160px;
  position: absolute;
  top: 0.5rem;
  right: 0;
}

.description {
    margin-left: 1.25rem;
  }

.theme-switch {
  display: inline-block;
  height: 34px;
  position: relative;
  width: 60px;
}

.theme-switch input {
  display:none;
}

.slider {
  background-color: #ccc;
  bottom: 0;
  cursor: pointer;
  left: 0;
  position: absolute;
  right: 0;
  top: 0;
  transition: .4s;
}

.slider:before {
  background-color: #fff;
  bottom: 0.25rem;
  content: "";
  width: 26px;
  height: 26px;
  left: 0.25rem;
  position: absolute;
  transition: .4s;
}

input:checked + .slider {
  background-color: var(--button-color);
}

input:checked + .slider:before {
  transform: translateX(26px);
}

.slider.round {
  border-radius: 34px;
}

.slider.round:before {
  border-radius: 50%;
}

#error, #success-msg {
  width: 40vw;
  margin: 0.125rem 0;
  font-size: 0.75rem;
  text-transform: uppercase;
  font-family: 'Jost';
  color: var(--secondary-color);
  }
}

  #success-msg {
  transition-delay: 3s;
  }

@media only screen and (max-width: 950px) {
  .logo {
    width: 8rem;
  }
  h1 {
    font-size: 1.75rem;
  }
  p {
    font-size: 0.7rem;
  }
  input, textarea, button {
    font-size: 0.65rem;
  }
  .description {
    font-size: 0.3rem;
    margin-left: 0.4rem;
  }
  button {
    width: 7rem;
  }
  .theme-switch-wrapper {
    width: 120px;
  }
  .theme-switch {
  height: 28px;
  width: 50px;
}

.theme-switch input {
  display:none;
}

.slider:before {
  background-color: #fff;
  bottom: 0.25rem;
  content: "";
  width: 20px;
  height: 20px;
  left: 0.25rem;
  position: absolute;
  transition: .4s;
}
  input:checked + .slider:before {
  transform: translateX(16px);
}

.slider.round {
  border-radius: 15px;
}

.slider.round:before {
  border-radius: 50%;
}

}

const toggleSwitch = document.querySelector('.theme-switch input[type="checkbox"]');

function switchTheme(e) {
    if (e.target.checked) {
        document.documentElement.setAttribute('data-theme', 'dark');
    }
    else {
        document.documentElement.setAttribute('data-theme', 'light');
    }    
}

toggleSwitch.addEventListener('change', switchTheme, false);


const name = document.getElementById('name');
const email = document.getElementById('email');
const message = document.getElementById('message');
const contactForm = document.getElementById('contact-form');
const errorElement = document.getElementById('error');
const successMsg = document.getElementById('success-msg');
const submitBtn = document.getElementById('submit');
  
const validate = (e) => {
  e.preventDefault();
 
  if (name.value.length < 3) {
    errorElement.innerHTML = 'Your name should be at least 3 characters long.';
    return false;
  } 
  
  if (!(email.value.includes('.') && (email.value.includes('@')))) {
    errorElement.innerHTML = 'Please enter a valid email address.';
    return false;
  } 

  if (!emailIsValid(email.value)) {
    errorElement.innerHTML = 'Please enter a valid email address.';
    return false;
  }

  if (message.value.length < 15) {
    errorElement.innerHTML = 'Please write a longer message.';
    return false;
  }

  errorElement.innerHTML = '';
  successMsg.innerHTML = 'Thank you! I will get back to you as soon as possible.'; 

  e.preventDefault();
  setTimeout(function () {
    successMsg.innerHTML = '';
    document.getElementById('contact-form').reset();
  }, 6000);

  return true;

}

const emailIsValid = email => {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

submitBtn.addEventListener('click', validate);

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