styles.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

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

.frame {
    flex: 0 0 25%;
    padding: 0.25rem;
    border: 1px solid rgb(198, 188, 188);
    border-radius: 15px;
    box-shadow: 3px 7px rgb(223, 222, 222);
}

.calculator {
    flex: 0 0 90%;
}

.screen {
    width: 100%;
    border: none;
    padding: 0.5rem;
    color: #333;
    background: rgb(223, 222, 222);
    font-size: 5rem;
    border-radius: 7px;
}

.buttons {
    display: flex;
    /* width: 100%; */
    flex-wrap: wrap;
    margin: 0.5rem 0.25rem;
    transition: all 0.5s linear;
}

button {
    flex: 0 0 25%;
    margin: 0.1rem 0rem;
    padding: 1rem 0.5rem;
    font-size:1.5rem;
    font-family: cursive;
    border: 1px solid rgb(198, 188, 188);
    border-radius: 5px;
    /* transition: all 2s ease; */
}

.btn {
    background: white;
    color: #333;
}

.btn:hover {
    background: rgb(239, 235, 235);
}

.btn-gray {
    background: rgb(245, 242, 242);
}

.btn-gray:hover {
    background: #fff;
}

.btn-red {
    background: rgb(243, 136, 136);
}

.btn-green {
    background: rgb(124, 236, 124);
}