#simple-popup {
    position: fixed;
    inset: 0;
    display: none;
    /* you already toggle this via JS */
    align-items: center;
    justify-content: center;
    background: rgba(0, 0, 0, 0.6);
    z-index: 9999;
}

/* popup box */
#simple-popup>div {
    background: #fff;
    padding: 24px;
    width: 100%;
    max-width: 340px;
    border-radius: 14px;
    position: relative;
    box-shadow: 0 25px 60px rgba(0, 0, 0, 0.25);
    box-sizing: border-box;
}

/* title */
#simple-popup h3 {
    margin: 0 0 16px;
    font-size: 18px;
    font-weight: 700;
    color: #0f172a;
    text-align: center;
}

/* close button */
#simple-popup button[type="button"] {
    position: absolute;
    top: 10px;
    right: 10px;

    width: 32px;
    height: 32px;

    border: none;
    border-radius: 50%;
    background: #f1f5f9;

    font-size: 18px;
    cursor: pointer;

    display: flex;
    align-items: center;
    justify-content: center;
}

/* form layout */
#simple-popup-form {
    display: flex;
    flex-direction: column;
}

/* inputs */
#simple-popup-form input {
    width: 100%;
    padding: 12px 14px;
    margin-bottom: 10px;

    border: 1px solid #e2e8f0;
    border-radius: 10px;

    font-size: 14px;
    outline: none;

    box-sizing: border-box;
    transition: 0.2s ease;
}

#simple-popup-form input:focus {
    border-color: #2563eb;
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.12);
}

/* submit button */
#simple-popup-form button[type="submit"] {
    width: 100%;
    padding: 12px;

    background: #2563eb;
    color: #fff;

    border: none;
    border-radius: 10px;

    font-weight: 600;
    cursor: pointer;

    transition: 0.2s ease;
}

#simple-popup-form button[type="submit"]:hover {
    background: #1d4ed8;
    transform: translateY(-1px);
}