style.css

*{
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.all{
  width: 100%;
  height: 100vh;
  background: rgb(255, 255, 255);
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
}

.all textarea{
  width: 40%;
  outline: none;
  resize: none;
  font-size: 16px;
  font-family: 'Roboto', sans-serif;
  color: #333;
  background: #fff;
  padding: 20px;
  box-sizing: border-box;
  border-radius: 5px;
  overflow: hidden;
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
  border: 0;
  box-shadow: -1px -1px 16px 10px rgba(0,0,0,0.21);
}

.all .btn{
  width: 40%;
  padding: 12px;
  min-width: 300px;
  background: #333;
  border: none;
  border-radius: 5px;
  color: #fff;
  font-size: 16px;
  font-family: 'Roboto', sans-serif;
  cursor: pointer;
  outline: none;
  transition: .3s;
  border-top-left-radius: 0;
  border-top-right-radius: 0;
  box-shadow: 0px 10px 16px 10px rgba(0,0,0,0.21);
}

.oscurecer{
  background: rgba(3, 3, 3, 0.632);
}