index.css

*{margin: 0;   
    padding: 0;   
    box-sizing: border-box;   
    outline: none;   
    border: none;   
    text-decoration: none;
}
body {
    font-family:  sans-serif;
    background-image: url(./imagens/criptografia-windows.jpg);
    background-repeat: no-repeat;
    background-size: cover;
    background-position: auto;
    height: 90%;
  }
  header{
      text-align: center;
      color: whitesmoke;
      width: 100%;
      background-color: transparent;
  }
  .conteiner{
  display: flex;
  justify-content: center;
  text-align: center;
  }
  #caixa{
    background-color: rgb(21, 3, 137);
    color: white;
    height: 100%;
    display: flex;
    flex-direction: column;
    justify-content: space-evenly;
    align-items: center;
    margin-top: 2%;
    padding: 1%;
    border-radius: 20px;
    text-shadow: 0px 0px 4px black;
    font-size: 15px;
  }
  #cifraCesar {
    text-align: center;
    display: none;
    flex-direction: column;
  }
  #incremento {
    transition: all 0.3s;
  }
  .mensagem textarea {
    width: 300px;
    height: 30px;
   
  }
  .mensagem2 textarea {
    width: 300px;
    height: 30px;
    
  }
  #cifraBase {
    text-align: center;
    display: none;
    flex-direction: column;
  }