style.css

* {
  box-sizing: border-box;
  transition: all .3s ease;
}

html, body {
  background: #eee;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}

main {
  width: calc(100% - 20px);
  max-width: 800px;
  margin: 0 auto;
  font-family: Helvetica, Arial, Sans, sans-serif;
}

h1, .user {
  margin: 0;
  padding: 10px 0;
  font-size: 32px;
  cursor: pointer;
}

.user {
  float: right;
}

.chat {
  content: '';
  width: 100%;
  height: calc(100vh - 250px); /* 165px */
  background: white;
  padding: 5px 10px;
}

.chat p {
  margin: 0 0 5px 0;
}

input, button {
  width: 100%;
  font: inherit;
  background: #fff;
  border: none;
  margin-top: 10px;
  padding: 5px 10px;
}

button:hover {
  cursor: pointer;
  background: #ddd;
}

@media all and (min-width: 500px) { /* Landscape */
  .chat {
    height: calc(100vh - 200px); /* 140px */
  }
  input {
    width: calc(100% - 160px);
  }
  button {
    float: right;
    width: 150px;
  }
}