#cuerpo {width: 99%;
        background: #33FFFF;
        font-size: 23px;
}
article {color: #000000;
    display: inline-block;
    margin: 0px 300px;
    font-size: 28px;
}
#Explicación { text-align: center;
            width: 87%;
            height: 50px;
            display: inline-block;
            border: 2px solid #9933FF;
            color: #FF0033;
 }
#Actividad { text-align: left;
            width: 87%;
            height: 650px;
            display: block;
 }
 #columna {width: 12%;
    height: 503px;
    text-align: center;
    display: inline-block;
    float: right;
}
#Siguiente { 
    border-radius: 10px;
    padding-bottom: initial;
    color: white;
    background-color: #9933FF;
    width: 100px;
    height: 100px;
    margin: 30px 10px;
    font-size: 25px;
}
.caja{color: #FF9900;
        width: 60px;
        height: 60px;
        font-size: 55px;
        text-align: center;
}
#Comprobar {
    border-radius: 10px;
    padding-bottom: initial;
    color: #FFFFFF;
    background-color: #9933FF;
    width: 100px;
    height: 50px;
    margin: 100px 10px;
    font-size: 14px;  
    float: center;
}
#cabecera {background: #FFCCFF;
        padding: 10px;
        width: 97%;
        height: 60px;
        border: 3px solid #9933FF;
}
img {width: 60px;
    display: inline-block;
}
#pie {background: #99FFFF;
    text-align: center;
    width: 114%;
    height: 30px;
    border: 5px solid #9933FF;
    color: #FF0033;
    margin-top: 70px;
}
#tabla2,#tabla3 {
        margin: 5px auto;
        background-color: palegreen;
}
.celda {color: blue;
        width: 90px;
        height: 90px;
        text-align: center;
        border: 2px solid #FF9900; 
        font-size: 60px;
        background-color: white;
    }.lleno {color: black;
        width: 90px;
        height: 90px;
        text-align: center;
        border: 2px solid #FF9900; 
        font-size: 60px;
        background-color: slategray;
    }
.raya {
        width: 105px;
        height: 10px;
        background-color:#000000;
    }
.sig {color: #000000;
        width: 30px;
        height: 105px;
        text-align: center;
        background-color: #33FFFF;
        border: 2px solid #33FFFF; 
        font-size: 60px;
}.tran {background-color: #33FFFF;
        width: 300px;
        height: 105px;
        border-top: 7px solid lightgreen;
        border-bottom: 7px solid lightgreen;
}#intentos{
    color: blue;
    font-size: 40px;
}#segundo{
    background-color: orange;
    margin: 0px 300px;
    display: none;
}#ultimo{
    background-color: orange;
    margin: 0px 300px;
    display:  none;
}#mensajeok{
        font-size: 70px;
        color: #9933FF;
        background-color: white;
        text-align: center;
        margin: 10px 200px;
        display: none;
}#mensajeno{
        font-size: 70px;
        color: #9933FF;
        background-color:white;
        text-align: center;
        margin: 10px 150px;
        display: none;
}#mensajefin{
        font-size: 50px;
        color: #9933FF;
        background-color:white;
        text-align: center;
        margin: 10px 150px;
        display: none;
}
<!--Para que no aparezcan las flechitas en la caja --> 
input[type=number]::-webkit-inner-spin-button, 
input[type=number]::-webkit-outer-spin-button { 
  -webkit-appearance: none; 
  margin: 0; 
}
//Firefox:
input[type="number"] {
-moz-appearance: textfield;
}
input[type="number"]:hover,
input[type="number"]:focus {
-moz-appearance: number-input;
}
input[type=number] {
    -moz-appearance:textfield;
}