style.css

body {
  font-family: 'Source Sans Pro', 'Helvetica Neue', Helvetica, Arial, sans-serif;
  margin: 3em 0;
  padding: 0;
  height: 100%;
}

.container {
  width: 50%;
  height: 500px;
  margin: auto;
  border: 1px solid #333;
}

h1 {
  font-size: 1.5em;
  font-weight: 600;
  letter-spacing: .3em;
  text-align: center;
  text-indent: .3em;
  text-transform: uppercase;
}

.rooms-list {
  width: 30%;
  height: 100%;
  float: left;
  padding: 0px 10px;
  border-right: 1px solid #333;
  overflow-y: auto;
}
.rooms-list ul {
  padding: 0;
}
.rooms-list ul li {
  border-bottom: 1px solid #f1f1f1;
  cursor: pointer;
  padding: 1em 0;
  position: relative;
  overflow: hidden;
  transition: background 300ms ease-out;
  border-left: 4px solid white;
}
.rooms-list ul li:hover {
  cursor: pointer;
  background-color: #fefff4;
  border-left: #eb5f3a 4px solid;
}

.chat-room {
  width: 65%;
  float: left;
  padding: 0px 5px;
  position: relative;
  height: 100%;
}
.chat-room h1 {
  border-bottom: 1px solid #f1f1f1;
}
.chat-room ul {
  overflow-y: auto;
  padding: 10px;
  height: 70%;
}
.chat-room ul li {
  list-style-type: none;
  padding: 5px;
}

#message {
  position: absolute;
  border-radius: 0;
  background-color: #FFFFFF;
  border-style: solid;
  border-width: 1px;
  border-color: #cccccc;
  box-shadow: inset 0 1px 2px rgba(0, 0, 0, 0.1);
  color: rgba(0, 0, 0, 0.75);
  display: block;
  font-family: inherit;
  font-size: 1rem;
  height: 2.05556rem;
  margin: 0 0 0.88889rem 0;
  padding: 0.44444rem;
  width: 95%;
  bottom: 0px;
}
#message:focus {
  outline: none;
}

#login-buttons {
  position: absolute;
  right: 250px;
  top: 50px;
}

@media (min-width: 480px) and (max-width: 1280px) {
  .container{
    width: 80%;
  }
  .chat-room{
    width: 60%;
  }
  .rooms-list{
    width: 30%;
  }
}
@media (max-width: 480px){
  .container{
    width: 100%;
  }
  .chat-room{
    width: 70%;
  }
  .rooms-list{
    width: 20%;
  }
}