html {
    font-size: 70%;
    box-sizing: border-box;
    background-color:  rgb(47, 71, 65);
}

*, *::before, *::after {
    margin: 0;
    padding: 0;
    box-sizing: inherit;
}

.calculator {
    border: 1.2px solid darkslategray;
    border-radius: 7px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 500px;
    background-color: white;
}

.calculator-screen {
    width: 100%;
    font-size: 5rem;
    height: 80px;
    border: none;
    background-color: black;
    color: white;
    text-align: right;
    padding-right: 20px;
    padding-left: 20px;
    border-radius: 7px 7px 0px 0px;
}

button {
    height: 70px;
    border-radius: 2px;
    border: 1px solid darkslategray;
    background-color: transparent;
    font-size: 3.2rem;
    color: black;
    background-image: linear-gradient(to bottom,transparent,transparent 50%);
    text-shadow: 0 1px rgba(255,255,255,.4);
    box-shadow: inset 0 0 0 1px rgba(255,255,255,.05), inset 0 1px 0 0 
        rgba(255,255,255,.05), inset 0 -1px 0 0 rgba(255,255,255,.05), 
        0 1px 0 0 rgba(255,255,255,.05);
}

button:hover {
    background-color:  rgb(211, 248, 237);
}

.operator {
    color: black;
    background-image: linear-gradient(to bottom,transparent,transparent 50%, rgba(0,0,0,.5));
    background-color: lightgray;
}

.all-clear {
    background-color: rgb(127, 197, 176);
    border-color: darkslategray;
    color: #fff;
}

.all-clear:hover {
    background-color: rgb(84, 129, 116);
}

.equal-sign {
    background-color: rgb(44, 201, 154);
    border-color: darkslategray;
    color: #fff;
    height: 100%;
    grid-area: 2 / 4 / 6 / 5;
}

.equal-sign:hover {
    background-color:rgb(23, 107, 82);
}

.calculator-keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 20px;
    padding: 20px;
}