style.css

body{
    background-color: antiquewhite;
}
.contenedor{
    width: 100%;
    height: 100%;
    margin: 0 auto;
    overflow: hidden;
    background-color: antiquewhite;
}
header{
    width: 100%;
    height: 100%;
    background-color: black;
}
.main{
    padding: 20px;
    float: left;
    box-sizing: border-box;
}
.copyright{
    text-align: center;
    color: black;
    font-size: 15px;
    margin: 80px  40px 20px;    
}
.titulo{
    font-size: 40px;
    color: whitesmoke;
    text-transform: uppercase;
    text-align: center;
    padding-left: 30px;
}
form{
    margin: 40px 0;
}
form textarea{
    float: left;
    margin: 10px 5px 15px 50px;
    padding: 15px 25px;
    width: 40%;
    resize: none;
    border-radius: 5px;

}
.caracteres{
    margin: 0 45px;
    padding: 5px;
    display: inline-block;
    font-size: 15px;
    color: red
}
#encriptar, #desencriptar, #copiar{
    width: 150px;
    margin: 50px 6px 10px 200px;
    padding: 15px 10px;
    font-size: 20px;
    background: orange;
    cursor: pointer;
    border-radius: 5px;
}
/*celulares*/
@media (max-width: 767px){
    header, .copyright{
        text-align: center;
    }
    form textarea{
        margin-left: 10px;
        width: 80%;
        position:static;
    }
    #encriptar, #desencriptar, #copiar{
        position:static; 
        width: 90%;
        margin: 5px 15px 5px 15px;
        padding: 10px;
    }
    .caracteres{
        display: inline-block;
        text-align: left;
        margin: 0 20px;
    }
}
/*para tablet*/
@media (min-width: 768px) and (max-width: 990px){
    header, footer{
        text-align: center;
    }
    form textarea{
        margin-left: 10px;
        margin-right: 10px;
        width: 0 auto;
    }
    #encriptar, #desencriptar, #copiar{
        position:relative; 
        width: 20%;
        margin: 35px 25px 5px  40px;
        display: inline-block;
        
    }
    .caracteres{
        display: inline-block;
        margin: 20px 15px 5px;
    }
    .copyright{
        display:inline-flex;
        
    }
}