style.css

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

body {
    margin: 10px;
    padding: 15px;
    display: flex;
    flex-direction: column;
    height: 90vh;
    justify-content: center;
    align-items: center;
    vertical-align: middle;
    background-color: hsl(0, 0%, 70%);
    background-image: url('images/banner.png');
    box-sizing: border-box;
}

.container {
    background-color: hsl(0, 0%, 15%);
    border-radius: 12px;
    max-width: 500px;
    width: 100%;
    padding: 10px 20px 20px;
    position: relative;
    box-shadow: 0 5px 10px rgba(0, 0, 0, 0.05);
}

.display {
    height: 80px;
    width: 100%;
    outline: none;
    border: none;
    padding: 20px;
    margin-bottom: 10px;
    border-radius: 10px;
    font-size: 3rem;
    text-align: right;
    background-color: hsl(0, 0%, 20%);
    color: white;
    pointer-events: none;
    display: flex;
}

.buttons {
    display: grid;
    grid-gap: 10px; 
    grid-template-columns: repeat(4, 1fr);
    border-radius: 50px;
}

.buttons button{
    padding: 10px;
    border-radius: 50px;
    border: none;
    color: white;
    font-size: 2rem;
    cursor: pointer;
}

button:hover{
    background-color: hsl(0, 0%, 40%);
}

.buttons button:active{
    transform: scale(0.99);
    background-color: hsl(0, 0%, 50%);
}

.operator{
    background-color: hsl(36.15,100%,52.16%);
}
.operator:hover{
    background-color: hsl(36.15,100%,62.16%);
}
.operator:active{
    background-color: hsl(36.15,100%,72.16%);
}

.other{
    background-color: hsl(0, 0%, 50%);
}
.other:hover{
    background-color: hsl(0, 0%, 60%);
}
.other:active{
    background-color: hsl(0, 0%, 70%); 
}

.numbers{
    background-color: hsl(0, 0%, 25%)
}
.numbers:hover{
    background-color: hsl(0, 0%, 35%);
}
.numbers:active{
    background-color: hsl(0, 0%, 45%); 
}

.links {
    border: none;
    outline: none;
    padding: 5px 5px;
    background-color: inherit;
    font-size: 16px;
    cursor: pointer;
    border-radius: 50px;
    color: black;
}

.links:hover {
    background-color: lightgray;
}

img {
    width: 30px;
    height: 30px;
}

#footer {
    text-align: center;
    position: relative;
    justify-content: baseline;
    align-items: center;
}