This is the finished CSS file for my blogpost https://medium.com/p/dcaa1323bec2/edit on responsiveness using Flexbox

company.css

body {
  margin: 0;
  font-family: "Hind Madurai", sans-serif;
}

body > div {
  min-height: 30vh;
  min-width: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}

.header {
  background-color: #f4511e;
  color: white;
  flex-direction: column;
}

.header-title {
  font-size: 50px;
}

.header-text {
  font-size: 20px;
}

.section {
  flex-flow: row wrap;
}

.section > div {
  padding: 30px;
}

.small-div {
  width: 200px;
}

.big-div {
  width: 800px;
}

.bg-grey {
  background-color: #f6f6f6;
}

.orange {
  color: #f4511e;
}

.div-title {
  font-size: 30px;
  font-weight: 600;
}

.contact-container {
  flex-flow: column;
  padding-top: 30px;
}

.contact-form {
  display: flex;
  flex-flow: row wrap;
  justify-content: center;
  width: 100%;
}

.contact-form > div {
  width: 500px;
  max-height: 300px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  margin: 30px;
}

.input-field {
  height: 40px;
  margin: 15px 0;
}

.contact-btn {
  margin-top: 15px;
  height: 40px;
  color: white;
  border: 0;
  background-color: #f4511e;
}