*, *::before, *::after {
    box-sizing: border-box;
    font-family: Segoe UI, sans-serif;
    font-weight: normal;   
}

body {
    padding: 0;
    margin: 0;
    background: linear-gradient(-45deg, #EE7752, #E73C7E, #23A6D5, #23D5AB);
    background-size: 400% 400%;
    -webkit-animation: Gradient 15s ease infinite;
	-moz-animation: Gradient 15s ease infinite;
	 animation: Gradient 15s ease infinite;
}

@-webkit-keyframes Gradient {
	0% {
		background-position: 0% 50%
	}
	50% {
		background-position: 100% 50%
	}
	100% {
		background-position: 0% 50%
	}
}

@-moz-keyframes Gradient {
	0% {
		background-position: 0% 50%
	}
	50% {
		background-position: 100% 50%
	}
	100% {
		background-position: 0% 50%
	}
}

@keyframes Gradient {
	0% {
		background-position: 0% 50%
	}
	50% {
		background-position: 100% 50%
	}
	100% {
		background-position: 0% 50%
	}
}

.calculator-grid {
    display: grid;
    justify-content: center;
    align-content: center;
    min-height: 100vh;
    grid-template-columns: repeat(4, 100px);
    grid-template-rows: minmax(120px, auto) repeat(5, 100px);
}

.calculator-grid > button {
    cursor: pointer;
    font-size: 2rem;
    border: 1px solid whitesmoke;
    outline: none;
    background-color: rgba(255, 255, 255, .55);

    /* Grow */
    display: inline-block;
    vertical-align: middle;
    transform: translateZ(0);
    box-shadow: 0 0 1px rgba(0, 0, 0, 0);
    backface-visibility: hidden;
    -moz-osx-font-smoothing: grayscale;
    transition-duration: 0.3s;
    transition-property: transform;
}

.calculator-grid > button:hover {
    background-color: rgba(255, 255, 255, 1);

    /* Grow */
    transform: scale(1.1);
}

.calculator-grid > button:active {
    

    /* Grow */
    transform: scale(1.2);
}

.span-two {
    grid-column: span 2;
}

.output {
    grid-column: 1 / -1;
    background-color: rgba(0, 0, 0, .55);
    display: flex;
    align-items: flex-end;
    justify-content: space-around;
    flex-direction: column;
    padding: 10px;
    word-wrap: break-word;
    word-break: break-all;
}

.output .previous-operand {
    color: rgba(255, 255, 255, .55);
    font-size: 1.5rem;
}

.output .current-operand {
    color: white;
    font-size: 2.5rem;
}

.modal-box {
    width: 100%;
    height: 100%;
    position: absolute;
    align-content: center;
    top: 0;
    display: none;
    justify-content: center;
    align-items: center;
    grid-template-columns: repeat(7, 1fr); 
    grid-auto-rows: 100px;
}