main.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: Arial, Helvetica, sans-serif;
    outline: none;
    background-color: #c6c3c4;
}
 
.flex-container {
    display: flex;
    height: 100vh;
      align-items: center;
    justify-content: center;
}

.flex-items {
    width: 30rem;
    height: 40rem;
    margin-top: 2rem;
    padding: 1rem;
    border-radius: 2rem;
    box-shadow: inset 6px 6px 13px rgb(255, 255, 255),
    inset -6px -6px 13px rgba(0, 0, 0, 0.5),inset -6px -6px 13px rgb(255, 255, 255),
    inset 6px 6px 13px rgba(0, 0, 0, 0.5);
} 

 input {
    font-size: 1.75rem;
    box-shadow: inset -6px -6px 13px rgb(255, 255, 255),
    inset 6px 6px 13px rgba(7, 0, 0, 0.5),6px 6px 13px rgb(255, 255, 255),
    inset -6px -6px 13px rgba(0, 0, 0, 0.5);
    border: none;
    border-radius: 1rem;
    padding: 1rem;
    text-align: end;
    color: rgb(0, 0, 0);
    margin: 2.5rem 0 2rem 1.5rem;
    font-weight: 700;
}

.one, .two, .thre, .four, .five {
    font-size: 1.7rem;
    width: 64px;
    height: 64px;
    margin: 0.5rem 0 1rem 2.2rem;
    box-shadow: inset 6px 6px 13px rgb(255, 255, 255),
    inset 6px 6px 13px rgba(7, 0, 0, 0.5);
    padding: 1rem;
    text-align: center;
    border: none;
    border-radius: 50%;
    font-weight: 600;
    cursor: pointer;
}

.del {
    font-size: 1.2rem;
    font-weight: 700;
}

.result {
    width: 10.5rem;
    border-radius: 25px;
}

.one:hover,.two:hover,.thre:hover,.four:hover,.five:hover {
    transition: 0.2s ease-out;
    background-color: red;
}