* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: sans-serif;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: #f4f4f9;
}

.calculator-wrapper {
    width: 320px; /* Keep wrapper width consistent */
    border: 2px solid #000;
}

/* Screen Styling */
.calculator-screen {
    background-color: #000;
    color: white;
    text-align: right;
    padding: 20px;
    min-height: 100px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    word-wrap: break-word;
    word-break: break-all;
}

.previous-operand {
    font-size: 1.2rem;
    color: rgba(255, 255, 255, 0.7);
    min-height: 20px;
}

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

/* Button Grid Setup */
.calculator-buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    background-color: #000; /* Fills gaps with black to act as borders */
    gap: 2px; /* Creates the border effect between buttons */
}

/* Button Styling */
.btn {
    border: none;
    outline: none;
    background-color: #d8d8d8; /* Light gray  */
    font-size: 1.5rem;
    padding: 20px;
    cursor: pointer;
    transition: background-color 0.1s;
}

.operator {
    background-color: #007bff; /* vibrant blue */
    color: white;
}

/* Grid Spanning */
.span-two {
    grid-column: span 2;
}

.span-three {
    grid-column: span 3;
}

/* Required Hover & Active States */
.btn:hover {
    background-color: #c4c4c4;
}

.operator:hover {
    background-color: #0056b3; /* darker blue */
}

.btn:active {
    background-color: #a6a6a6;
    transform: scale(0.98); /* Slight visual push effect */
}