index.css

@import url('https://fonts.googleapis.com/css2?family=Ubuntu+Mono&display=swap');

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Ubuntu Mono', monospace;
}

body{
    background: linear-gradient(white, blue) no-repeat;
    width: 100%;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 20px;
}

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

header h2{
    text-align: center;
    margin: 16px;
    font-size: 2.4rem;
}

.container{
    background-color: black;
    text-align: center;
    width: 55%;
    border: 2px solid black;
    border-radius: 20px 20px 0 0;
    padding: 32px;
    color: white;
}

.container p{
    margin: 8px;
    font-size: 1.3rem;
}

.container label{
    margin: 10px;
    font-size: 1.2rem;
}

.container input{
    cursor: pointer;
}

.btn, .btn-main{
    background-color: aqua;
    padding: 6px;
    margin: 12px;
    font-size: 1.1rem;
    border: 2px solid black;
    border-radius: 12px;
    width: 10vw;
    cursor: pointer;
    outline: none;
}

.btn{
    background-color: pink;
}

.btn:hover, .btn-main:hover{
    background-color: gray;
    color: white;
}

#cgpa, #percent{
    background-color: black;
    color: white;
    width: 55%;
    text-align: center;
    display: none;
    border-radius: 0 0 20px 20px;
    padding: 15px;
}

#cgpa h2, #percent h2{
    font-size: 1.8rem;
    margin: 7px;
}

#cgpa label, #percent label{
    font-size: 1.3rem;
    padding: 8px;
}

#cgpa input, #percent input{
    width: 50%;
    font-size: 1.1rem;
    padding: 3px;
    margin: 6px;
    outline: none;
}

@media only screen and (max-width: 1445px)  {
    .container, #cgpa, #percent{
        width: 65%;
    }
}

@media only screen and (max-width: 1229px)  {
    .container, #cgpa, #percent{
        width: 75%;
    }
}

@media only screen and (max-width: 1070px)  {
    .container, #cgpa, #percent{
        width: 85%;
    }
}

@media only screen and (max-width: 949px)  {
    .container, #cgpa, #percent{
        width: 95%;
    }

    header h2{
        font-size: 2rem;
    }

    .conatiner{
        padding: 28px;
    }

    .container label{
        margin: 6px;
    }

    .btn, .btn-main{
        width: 30vw;
    }
}