body {
    font-family: Arial, sans-serif;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #f0f0f0;
    margin: 0;
}

.timer-container {
    background-color: white;
    padding: 2rem;
    border-radius: 10px;
    box-shadow: 0 4px 8px rgba(0,0,0,0.1);
    text-align: center;
}

.input-group {
    margin: 1rem 0;
}

input[type="number"] {
    width: 60px;
    padding: 0.5rem;
    font-size: 1rem;
    border: 1px solid #ccc;
    border-radius: 5px;
    text-align: right;
}

#start-btn {
    padding: 0.75rem 1.5rem;
    font-size: 1rem;
    color: white;
    background-color: #007bff;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s;
}

#start-btn:hover {
    background-color: #0056b3;
}

#reset-btn {
    padding: 0.75rem 1.5rem;
    font-size: 1rem;
    color: #333;
    background-color: #f8f9fa;
    border: 1px solid #ccc;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s;
    margin-left: 0.5rem;
}

#reset-btn:hover {
    background-color: #e2e6ea;
}

#display-time {
    margin-top: 1.5rem;
    font-size: 3rem;
    font-weight: bold;
    color: #333;
}

#flash-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: white;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.1s ease-in-out;
    z-index: 1000; /* 他の要素の上に表示 */
}

#flash-overlay.active {
    visibility: visible;
    animation: flash 0.3s infinite;
}

@keyframes flash {
    0% { opacity: 0; }
    50% { opacity: 1; }
    100% { opacity: 0; }
}

#back-to-top-btn {
    margin-top: 2rem;
    padding: 0.75rem 1.5rem;
    font-size: 1rem;
    color: white;
    background-color: #6c757d;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s;
}

#back-to-top-btn:hover {
    background-color: #5a6268;
}