.calculator {
    text-align: center;
    margin: 0 auto;
    padding-top: 10px;
    width: 400px;
}

.calculator-screen {
    width: 100%;
    height: 100px;
    background-color: #252525;
    color: #fff;
    text-align: right;
    font-size: 36px;
    border: none; 
    padding: 0 20px;
    box-sizing: border-box;
}

.calculator-keys {
    width: 100%;
}

.row {
    display: flex;
}

button {
    height: 80px;
    background-color: gray;
    border: 1px solid black;
    font-size: 32px;
    color:#fff;
    width: 25%;
    outline: none;
}

.all-clear, .zero-btn{
    width: 50%;
}

.operator, .equal-sign{
    background-color: orange;
}

button:hover {
    background-color: darkgrey;
}

.operator:hover, .equal-sign:hover{
    background-color: darkorange;
}


.title-name {
    font-size: 2.75rem;
    font-weight: 900;
    letter-spacing: 0.4rem;
    color: #ffc107;
    text-align: center;
}

.text-uppercase {
    text-transform: uppercase;

}

/* button another theme */
.btn-theme-page {
    text-align: right;

}

a {
    text-decoration: none;
    display: inline-block;
    padding: 8px 16px;
  }
  
  a:hover {
    background-color: #ddd;
    color: orange;
  }
  
  .btn-theme-1 {
    background-color: orange;
    color: white;
  }
  
  .btn-theme-2 {
    background-color: #f1f1f1;
    color: orange;
  }