Edit in JSFiddle

body {
  display: flex;
  flex-direction: column;
  background-color: #D8CCAF;
  margin: 0;
  font-family: "Lucida Grande", "Lucida Sans Unicode", "Lucida Sans", "DejaVu Sans", Verdana, sans-serif;
}

#header {
  height: 80px;
  line-height: 80px;
  text-align: center;
  background-color: #575757;
  font-size: 2em;
  color: #ffffff;
  padding: 15px;
}

#header h2 {
  margin: 0;
}

#nav {
  color: white;
  height: 50px;
  line-height: 50px;
  text-align: center;
  background-color: #E49A2F;
  overflow: auto;
}

#nav a {
  color: white;
}

#content {
  display: flex;
  flex-wrap: wrap;
  flex-direction: column;
  justify-content: center;
  flex-grow: 1;
  background-color: #D8CCAF;
}

#content_left,
#content_right {
  box-sizing: border-box;
  flex-grow: 0;
  flex-shrink: 0;
  flex-basis: calc(50% - 40px);
  margin: 10px;
  background-color: #F7F5EE;
  border: 1px;
  padding: 10px;
  border-radius: 25px;
  border-color: #AC9675;
}

@media screen and (min-width: 769px) {
  #content {
    flex-direction: row;
  }
}

.kuva {
  width: calc(100% - 20px);
  max-width: 350px;
  height: auto;
  display: block;
  margin: 0 auto;
}

#footer {
  height: 50px;
  line-height: 50px;
  background-color: #E49A2F;
  color: #ffffff;
  text-align: center;
  font-size: 12px;
}