Here is a basic CSS file to style your calculator:

```css
/* General Styles */
body {
    font-family: Arial, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background-color: #f0f0f0;
}

/* Calculator Container */
.calculator {
    width: 320px;
    background-color: #fff;
    border-radius: 10px;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.1);
    overflow: hidden;
}

/* Display Screen */
.calculator-screen {
    width: 100%;
    height: 80px;
    background-color: #272727;
    color: #fff;
    border: none;
    padding: 20px;
    font-size: 2rem;
    text-align: right;
    box-sizing: border-box;
}

/* Calculator Buttons */
.button {
    width: 25%;
    height: 60px;
    background-color: #f1f1f1;
    border: 1px solid #ddd;
    float: left;
    outline: none;
    font-size: 1.5rem;
    cursor: pointer;
    transition: background-color 0.3s;
}

.button:hover {
    background-color: #e0e0e0;
}

.button:active {
    background-color: #ccc;
}

.button.operation {
    background-color: #ff9500;
    color: #fff;
}

.button.operation:hover {
    background-color: #e08900;
}

.button.double {
    width: 50%;
}

.clear {
    background-color: #ff3b30;
    color: #fff;
}

.clear:hover {
    background-color: #e32e24;
}
```

This stylesheet provides a clean layout for your calculator. It styles the calculator's body, display screen, and buttons, including hover effects and active states for a more interactive user experience. You can adjust colors, sizes, and other properties to fit your design preferences.