index.css

*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

.overlay{
    padding-top: 100px;

    display: grid;
    place-items: center;
}

.box{
    display: grid;
    place-items: center;
    width: 500px;
    height: 350px;

    background: linear-gradient(to right, #32de84, #00FA9A);
    box-shadow: 0 0 7px 7px rgba(0, 0, 0, 0.4);
    border-radius: 20px;
    padding: 10px;
}

.change{
    width: 40px;
    height: 40px;
    background-color: transparent;
    color: rgb(255, 255, 255);
    border: none;
    border: 1px solid rgb(255, 255, 255);
    font-weight: bold;
    font-size: 25px;
    cursor: pointer;
}

.inline{
    display: flex;
    margin-bottom: 20px;
}

.m, 
.ft,
.cm, 
.in,
.m-2, 
.ft-2,
.cm-2, 
.in-2{
    width: 200px;
    padding: 15px;
    font-size: 17px;
    font-family: sans-serif;
    background-color: transparent;
    border: 1px solid rgb(255, 255, 255);
    color: rgb(255, 255, 255);
    font-weight: 600;
}

.m:focus,
.ft:focus,
.cm:focus,
.in:focus,
.m-2:focus,
.ft-2:focus,
.cm-2:focus,
.in-2:focus{
    outline: none;
}

.m::placeholder,
.ft::placeholder,
.cm::placeholder,
.in::placeholder,
.m-2::placeholder,
.ft-2::placeholder,
.cm-2::placeholder,
.in-2::placeholder{
    color: rgb(255, 255, 255);
    font-weight: 500;
}

.m-2,
.cm-2,
.ft,
.in{
    margin-left: 20px;
    cursor: default;
}

.convert{
    padding: 10px 20px;
    border: none;
    border: 1px solid rgb(255, 255, 255);
    background-color: transparent;
    color: rgb(255, 255, 255);
    font-weight: 800;
    font-size: 17px;
    cursor: pointer;
}

.hidden-1,
.hidden-2{
    display: none;
}

@media (max-width: 499px) {
    .box{
        width: 300px;
    }

    .m, 
    .ft,
    .cm, 
    .in,
    .m-2, 
    .ft-2,
    .cm-2, 
    .in-2{
        width: 120px;
    }
}