style.css

*{
    margin:0;
    padding:0;
}
h1,h2,h3,h4,h5,h6{
    margin:0;
    padding:0;
}
body{
    background: #eee;
    box-sizing: border-box;
    font-size: 14px;
    height: 100vh;
    display: grid;
    align-items: center;
    font-family: JetBrains mono;
}
:focus{
    outline: none;
}
.wrapper {
    background: #ffa84e;
    width: 768px;
    margin: 0 auto;
    height: 61vh;
    display: grid;
    align-items: center;
    border-radius: 9px;
}

form#form-group .inputNum {
    font-size: 24px;
    overflow: hidden;
}

input#hexadecimal {
    width: 96.7%;
    font-size: 34px;
    padding: 6px 12px;
    margin: 20px 0px;
    margin-top: 10px;
    border-radius: 11px;
    border: none;
    font-family: JetBrains mono;
}

form#form-group h4 {
    color: #fff;
    font-size: 33px;
    margin-bottom: 27px;
}

form#form-group {
    padding: 14px;
}

.outputNum h3 {
    font-size: 36px;
    font-weight: 400;
    border-bottom: 1px solid;
    background: #fff;
    border-radius: 10px;
    padding: 2px 10px;
    cursor:text;
}

.outputNum {
    margin-bottom: 40px;
}

.outputNum h3 span {
    visibility:hidden;
}

.submitBtn {
    text-align: center;
}

.submitBtn #btn {
    border: none;
    font-size: 30px;
    width: 150px;
    background: #042bff;
    color: #fff;
    font-weight: bold;
    font-family: JetBrains mono;
    padding: 8px;
    margin: 0 auto;
    border-radius: 5px;
    cursor: pointer;
    text-transform: uppercase;
}