.theme-toggle-btn {
    position: fixed;
    bottom: 10px;
    right: 10px;
    z-index: 10000; /* high enough to stay on top */
    padding: 0.33rem 0.33rem;
    font-size: 1.2rem;
    background: midnightblue;
    border: none;
    cursor: pointer;
    border-radius: 6px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.15);
    user-select: none;
}