*, *::before, *::after {
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
    margin: 0;
    background: linear-gradient(to right, #4facfe, #00f2fe);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    overflow: hidden; /* Prevent scrollbar due to subtle movements */
}

.calculator {
    display: grid;
    grid-template-columns: repeat(4, 100px);
    grid-template-rows: minmax(120px, auto) repeat(5, 80px);
    background-color: rgba(0, 0, 0, .75);
    border-radius: 15px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.4);
    overflow: hidden; /* Ensure rounded corners for internal elements */
}

.calculator button {
    cursor: pointer;
    font-size: 2rem;
    border: 1px solid rgba(255, 255, 255, 0.1);
    outline: none;
    background-color: rgba(255, 255, 255, .2);
    color: white;
    transition: background-color 0.2s ease-in-out, transform 0.1s ease-in-out; /* Transitions for hover and active */
}

.calculator button:hover {
    background-color: rgba(255, 255, 255, .3);
}

.calculator button:active {
    transform: scale(0.95); /* Slight shrink effect on click */
}

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

.display {
    grid-column: 1 / -1; /* Spans all columns */
    background-color: rgba(0, 0, 0, .85);
    display: flex;
    flex-direction: column;
    align-items: flex-end;
    justify-content: space-around;
    padding: 15px;
    word-wrap: break-word; /* Allows long numbers to wrap */
    word-break: break-all; /* Breaks words if necessary */
    border-bottom: 1px solid rgba(255, 255, 255, 0.1); /* Separator */
}

.display .previous-operand {
    color: rgba(255, 255, 255, .75);
    font-size: 1.5rem;
    text-align: right;
    width: 100%; /* Ensure it takes full width */
    min-height: 1.5em; /* Reserve space */
}

.display .current-operand {
    color: white;
    font-size: 3rem;
    text-align: right;
    width: 100%; /* Ensure it takes full width */
}

/* Specific styling for operation buttons for visual distinction */
button[data-operation], button[data-equals] {
    background-color: rgba(255, 165, 0, 0.6); /* Orange tint for operations */
}

button[data-operation]:hover, button[data-equals]:hover {
    background-color: rgba(255, 165, 0, 0.8);
}