Edit in JSFiddle

 <form action="action_page.php">
  <div class="container">
    <p>Please fill in this form to create an account.</p>

    <label for="FullName"><b>Full Name</b></label>
    <input type="text" placeholder="Enter your Full Name" name="FullName" required>

    <label for="email"><b>Email</b></label>
    <input type="text" placeholder="Enter Email" name="email" required>

    <label for="password"><b>Password</b></label>
    <input type="password" placeholder="Enter Password" name="password" required>

    <label for="password-repeat"><b>Repeat Password</b></label>
    <input type="password" placeholder="Repeat Password" name="password-repeat" required>

    <p>By creating an account you agree to our <a href="#">Terms & Privacy</a>.</p>
    <button type="submit" class="RegisterButton">Register</button>

  <div class="container signin">
    <p>Already have an account? <a href="#">Sign in</a>.</p>
/* Add padding to containers */
.container {
  padding: 16px;

/* Full-width input fields */
input[type=text], input[type=password] {
  width: 100%;
  padding: 15px;
  margin: 5px 0 22px 0;
  display: inline-block;
  border: none;
  background: #f1f1f1;

input[type=text]:focus, input[type=password]:focus {
  background-color: #ddd;
  outline: none;

/* Overwrite default styles of hr */
hr {
  border: 1px solid #f1f1f1;
  margin-bottom: 25px;

/* Set a style for the submit/register button */
  background-color: #4CAF50;
  color: white;
  padding: 16px 20px;
  margin: 8px 0;
  border: none;
  cursor: pointer;
  width: 100%;
  opacity: 0.9;

.RegisterButton:hover {

/* Add a blue text color to links */
a {
  color: dodgerblue;

/* Set a grey background color and center the text of the "sign in" section */
.signin {
  background-color: #f1f1f1;
  text-align: center;