styles.css

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

.main {
    height: 100vh;
    background-color: #E9D5DA;
    display: flex;
    align-items: center;
    justify-content: center;
}

.inner-main {
    background-color: #827397;
    border-radius: 30px;
    padding: 40px;
}

.result input {
    width: 150px;
    height: 60px;
    float: left;
    border-radius: 10px 0 0 0;
    border: none;
    padding: 5px;
    font-size: 11pt;
}

.result input:focus {
    outline: none;
}

.result button {
    width: 50px;
    height: 60px;
    float: left;
    font-size: 11pt;
    border-radius: 0 10px 0 0;
    border: none;
}

.keypad .row button {
    width: 50px;
    height: 60px;
    float: left;
    border: none;
}

.result button:hover,
.keypad .row button:hover {
    cursor: pointer;
    background-color: rgba(50, 50, 168, 0.7);
    color: white;
}


.first-child {
    border-radius: 0 0 0 10px;
}

.last-child {
    border-radius: 0 0 10px 0;
}

@media screen and (min-width: 450px) {
    .result input {
        width: 225px;
    }

    .result button {
        width: 75px;
    }

    .keypad .row button {
        width: 75px;
    }
}

@media screen and (min-width: 992px) {
    .result input {
        width: 375px;
    }

    .result button {
        width: 125px;
    }

    .keypad .row button {
        width: 125px;
    }
}