CSS

transparent input box

input{
  background: transparent;
  color: #FFF;
  top: 40vh;
  position: relative;
  border: 1px solid #FFF;
  display: inline-block;
}

css transparent input text box

background-color:rgba(0, 0, 0, 0);

transparent input box

input[type="text"]
{
    background: transparent;
    border: none;
}