body {
    background-color: #212121;
    color: #e5e5e5;
    text-align: center;
    font-family: 'Arial', sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
}

.main-box {
    width: 100%;
    max-width: 320px;
    padding: 15px;
}

.container {
    border: 2px solid #7e7d7d;
    border-radius: 12px;
    background-color: #2b2626;
    padding: 20px;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.4);
}

.container h2 {
    margin-bottom: 20px;
}

.container div {
    display: flex;
    justify-content: space-between;
    margin: 0;
}

.container div span {
    flex: 1;
    margin: 8px;
    padding: 15px;
    border: 1px solid rgba(0, 0, 0, 0.2);
    border-top: 1px solid rgba(255, 255, 255, 0.2); /* Highlight for glassy effect */
    border-radius: 8px;
    cursor: pointer;
    font-size: 28px;
    font-weight: bold;
    color: #fff;
    text-shadow: 0 0 2px rgba(0,0,0,0.5); /* Text pop effect */
    transition: all 0.15s ease;
    background-color: #3c3c3c; /* Darker base for number buttons */
    box-shadow: 0 2px 3px rgba(0, 0, 0, 0.3);
    text-align: center;
}

.container div span:active, .active-key {
    transform: scale(0.95);
    box-shadow: 0 1px 2px rgba(0, 0, 0, 0.2);
}

.output {
    border: 1px solid #5b5a5a;
    height: 70px;
    border-radius: 8px;
    background-color: #464545;
    margin: 0 5px 20px 5px !important;
    cursor: text;
    color: #fff;
    text-align: right !important;
    font-size: 40px;
    justify-content: flex-end !important;
    align-items: center;
    padding: 10px;
    box-shadow: inset 0px 2px 5px rgba(0, 0, 0, 0.3);
    overflow-x: auto;
    white-space: nowrap;
}

/* --- Glassy Button Styles --- */

.equal {
    background-image: linear-gradient(to bottom, #4dabf7, #1e88e5) !important;
}

.AC {
    background-image: linear-gradient(to bottom, #ff7063, #e53935) !important;
    font-size: 24px !important; /* Making AC button font size smaller */
    padding:8px !important;
}

.percentage, .back, .division, .mul, .sub, .add {
    background-image: linear-gradient(to bottom, #ffab40, #ff9100) !important;
}

i {
    font-size: 20px;
}

.back-btn {
  position: relative;
  display: inline-block;
  text-decoration: none;
  background-color: #ff9100;
  height: 30px;
  width: 50px;
  border-radius: 5px;
}

.back-btn i{
    text-align: center;
  justify-content: center;
  position: relative;
  top:7px;
}
.back-btn::after {
  content: "Go Back to Project List Page";
  position: absolute;
  top: 120%; /* below the icon */
  left: 50%;
  transform: translateX(-80%) translateY(-230%);;
  /* transform: translateY(-250%); */
  background-color: #333;
  color: #fff;
  padding: 6px 10px;
  border-radius: 5px;
  font-size: 14px;
  white-space: nowrap;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease;
  z-index: 999;
}

.back-btn:hover::after {
  opacity: 1;
}

/* --- Responsive Design --- */

@media (max-width: 400px) {
    .container div span {
        font-size: 24px;
        padding: 12px;
    }
    .AC {
        font-size: 20px;   /* Adjust AC button for smaller screens */
    }
    .output {
        font-size: 36px;
    }
}