html {
    box-sizing: border-box;
}
*, *:before, *:after {
    box-sizing: inherit;
}
body {
    padding: 0;
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: #222;
}
.wrapper {
    width: 400px;
    background-color: #333;
    border-radius: 10px;
    padding: 20px;
}
.calculator-screen {
    background-color: #000;
    border-radius: 5px;
    margin-bottom: 15px;
}
.calculator-screen-input {
    width: 100%;
    background-color: #000;
    color: #fff;
    border: none;
    text-align: right;
    font-size: 2.5rem;
    padding: 20px 15px;
    border-radius: 5px;
}
.calculator-buttons {
    display: flex;
    flex-wrap: wrap;
}

.calculator-button {
    width: 25%;
    height: 70px;
    font-size: 1.3rem;
    background-color: #e0e0e0;
    border: 1px solid #ccc;
    cursor: pointer;
}

.operator {
    background-color: #f5a623;
    color: #fff;
}

.placeholder {
    background-color: transparent;
    border: none;
}
.placeholder {
    background-color: #e0e0e0;
    border: 1px solid #ccc;
    cursor: default;
}

.calculator-button:hover {
    background-color: #d0d0d0;
}

.operator:hover {
    background-color: #e09515;
}

.calculator-button:active {
    background-color: #bbb;
}

.operator:active {
    background-color: #c9880e;
}

.placeholder:hover, .placeholder:active {
    background-color: #e0e0e0;
}
.two-wide {
    width: 50%;
}
.three-wide {
    width: 75%;
}