main.css

* {
  box-sizing: border-box;
}
html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
}
a {
  text-decoration: none;
}
.btn {
  display: inline-block;
  padding: 0 5px;
  text-decoration: none;
  cursor: pointer;
  border-radius: 4px;
  background: white;
  border: 1px solid silver;
  color: black;
  height: 37px;
  line-height: 37px;
  vertical-align: top;
  font-size: 12px;
}
input[type='text'],
input[type='email'],
input[type='password'],
textarea {
  border-radius: 4px;
  height: 37px;
  padding: 10px;
  border: 1px solid silver;
}
.container {
  width: 100%;
  height: 100%;
}
@media screen and (min-width: 800px) {
  .container {
    width: 800px;
    margin: 0 auto;
  }
}
.input-group {
  margin-bottom: 15px;
}
.input-group label {
  width: 25%;
  display: inline-block;
}
.input-group input {
  width: 70%;
}

#join {
  float: right;
}
.profile-wrap {
  width: 100%;
  display: inline-block;
  vertical-align: top;
  margin: 10px 0;
}
@media screen and (min-width: 800px) {
  .profile-wrap {
    width: 290px;
    margin-bottom: 0;
  }
}
.profile {
  text-align: left;
  padding: 10px;
  margin-right: 10px;
  border-radius: 4px;
  border: 1px solid silver;
}
.user-name {
  font-weight: bold;
  font-size: 18px;
}

.timeline {
  margin-top: 10px;
  width: 100%;
  display: inline-block;
  border-radius: 4px;
  vertical-align: top;
}
@media screen and (min-width: 800px) {
  .timeline {
    width: 500px;
  }
}
.error-message {
  color: red;
  font-weight: bold;
}
#join-form {
  padding: 10px;
  text-align: center;
}
footer {
  text-align: center;
}