style.css

* {
    box-sizing: border-box;
    font-family: roboto, sans-serif;
}

.calculator {
    width: 350px;
    height: 550px;
    background: #3a4655;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    box-shadow: 0 8px 50px -7px black;
    border: solid 1px #3A4655;

    .calculator-screen {
        height: 30%; /* 165px */
        padding: 20px;

        .operation {
            height: 35%;
            padding-top: 15px;
            padding-left: 5px;
            padding-right: 5px;
            border-bottom: #727B86 dotted 1px;
            color: #727B86;
            direction: rtl;
            font-size: 21px;
        }

        input.result {
            padding: 25px 10px 5px;
            font-size: 45px;
            color: #FFF;
            text-align: end;
            background: inherit;
            width: 100%;
            border: none;
            margin-top: 10px;
        }
    }

    .calculator-buttons {
        height: 70%;
        width: 100%;

        .buttons-row {
            height: 19.6%;
            width: 100%;
            margin-bottom: 2px;
            display: flex;

            .button {
                border: none;
                height: 100%;
                width: 24.5%;
                margin-left: 2px;
                background: #404D5E;
                vertical-align: middle;
                text-align: center;
                font-size: 25px;
                color: #FFF;
                padding-top: 23px;
                cursor: pointer;
                outline: none;
                transition: 0.2s all;

                /* Adding transformation when the button is active */
                &:active {
                    transform: scale(0.98);
                    /* Scaling button to 0.98 to its original size */
                    box-shadow: 3px 2px 22px 1px rgba(0, 0, 0, 0.24);
                    /* Lowering the shadow */
                }

                &:hover {
                    background: red;
                    box-shadow:
                            0 0 60px 2px red,  /* inner white */
                            0 0 100px 3px red, /* middle magenta */
                            0 0 140px 5px red; /* outer cyan */
                }
            }
        }
    }
}