styles.css

html {
    margin: none;
    height: 100%;
    background-image: linear-gradient(to bottom right, #65dd93  0%, #54aabb 100%);
}

body {
    position: absolute;
    width: 98%;
    height: 98%;
    /* border: solid black 2px; */
}

.heading {
    position: relative;
    text-align: center;
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    font-size: 30px;
    margin-top: 20px;
    letter-spacing: 0.4em;
    margin: 5px 0px;
    /* border: solid black 2px; */

}

.maintext{
    position: relative;
    top:25%;
    text-align: center;
    font-family:'Segoe UI', Tahoma, Geneva, Verdana, sans-serif ;
    font-size: 20px;
    letter-spacing: 0.1em;
    /* border: solid black 2px; */
}

input::-webkit-outer-spin-button,
input::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}

.mainbody{
    display: flex;
    flex-flow: row wrap;
    align-items: center;
    justify-content: center;
    position: relative;
    text-align: center;
    top:25%;
    /* border: solid black 2px; */
}

#inputNum{
    border: solid rgb(51, 51, 51) 2px;
    border-radius: 10px;
    box-shadow: 0px 0px 8px rgb(156, 156, 156);
    background-color: rgb(221, 220, 220);
    position: relative;
    text-align: center;
    outline: none;
    caret-color: rgba(0, 0, 0, 0.233);
    font-family: monospace;
    font-size: 30px;
    font-weight: lighter;
}
#inputNum::placeholder{
    font-size: 20px;
    text-align: center;
    vertical-align:middle;
    transform: translate(2%,-13%);
}


.submit {
    margin: 5px;
    padding: 10px;
    width: 100px;
    font-family: monospace;
    font-size: 16px;
    text-align: center;
    text-transform: uppercase;
    transition: 0.5s;
    background-size: 200% auto;
    color: white;            
    box-shadow: 0 0 8px rgb(158, 158, 158);
    border-radius: 10px;
    display: block;
    background-image: linear-gradient(to right, #283048 0%, #859398  51%, #283048  100%);
    letter-spacing: 0.2em;
 }

 .submit:hover {
   background-position: right center;
   color: #fff;
   text-decoration: none;
 }

 @media (max-width: 500px) {
     .heading{
        font-size: 20px;
        letter-spacing: 0.2em;
        font-weight: 400;
     }
     .maintext{
         font-size: 15px;
     }
     #inputNum{
        font-size: 24px;
        width: 240px;
        height: 33px;
    }
    #inputNum::placeholder{
        font-size: 16px;
    }
    .submit{
        height: 40px;
    }
     
 }