.formula-item {
    width: 40px;
    height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: #e0e0e0;
    border-radius: 4px;
    font-weight: bold;
    transition: all 0.3s ease;
}
.formula-item.selected {
    background-color: #4caf50;
    color: white;
    transform: scale(1.1);
}