:root {
    --primary-color: #C0392B;  /* Pomegranate Red (Đỏ lựu) */
    --secondary-color: #E67E22; /* Carrot Orange (Cam cà rốt) */
    --dark-text: #34495E;     /* Wet Asphalt (Xanh đậm/Xám) */
    --light-text: #ECF0F1;    /* Clouds (Trắng mây) */
    --light-bg: #F8F9FA;      /* Nền xám rất nhạt */
    --card-bg: #FFFFFF;       /* Nền thẻ (Trắng) */
    --border-color: #DEE2E6;    /* Màu viền (Xám nhạt) */
    --success-color: #27AE60;   /* Xanh lá cây */
    --danger-color: #95A5A6;    /* Xám (cho nút "Xóa") */

    /* Biến dùng chung cho giao diện */
    --header-height: 65px;
    --border-radius-sm: 6px;
    --border-radius-md: 10px;
    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 5px 15px rgba(0, 0, 0, 0.08);
    --transition-speed: 0.3s;

    /* Màu cho các cấp độ HSK (dùng cho viền thẻ) */
    --level-color-1: #2980B9;
    --level-color-2: #27AE60;
    --level-color-3: #F1C40F;
    --level-color-4: #E67E22;
    --level-color-5: #E74C3C;
    --level-color-6: #8E44AD;
    --level-color-7: #34495E;
    --level-color-8: #7F8C8D;
    --level-color-9: #D35400;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Poppins', system-ui, -apple-system, 'Segoe UI', Roboto, 'Microsoft YaHei', 'SimHei', sans-serif;
    background-color: var(--light-bg);
    color: var(--dark-text);
    line-height: 1.7;
    font-size: 16px;
    -webkit-font-smoothing: antialiased;
}

h2, h3 {
    color: var(--dark-text);
    margin-bottom: 1.2rem;
    padding-bottom: 0.5rem;
    border-bottom: 2px solid var(--border-color);
    font-weight: 600;
}

h2 {
    font-size: 2rem;
}

h3 {
    font-size: 1.5rem;
    margin-top: 2.5rem;
}

header {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: var(--header-height);
    background-color: var(--card-bg);
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0 2rem;
    box-shadow: var(--shadow-sm);
    z-index: 1000;
}

.logo {
    font-size: 1.6rem;
    font-weight: 700;
    color: var(--dark-text);
    display: flex;
    align-items: center;
}

.logo::before {
    content: '笔';
    font-family: 'SimHei', sans-serif;
    background-color: var(--primary-color);
    color: var(--light-text);
    font-size: 1.25rem;
    margin-right: 0.75rem;
    font-weight: normal;
    width: 36px;
    height: 36px;
    display: inline-flex;
    justify-content: center;
    align-items: center;
    border-radius: 50%;
    transform: rotate(-10deg);
}

nav#main-nav {
    display: flex;
    height: 100%;
}

.nav-tab {
    font-family: 'Poppins', sans-serif;
    font-size: 1rem;
    font-weight: 500; /* Giảm độ đậm */
    color: var(--dark-text);
    background: none;
    border: none;
    padding: 0 1.25rem;
    height: 100%;
    cursor: pointer;
    border-bottom: 4px solid transparent;
    transition: all var(--transition-speed) ease;
    position: relative;
}

.nav-tab:hover {
    color: var(--primary-color);
    background-color: var(--light-bg);
}

.nav-tab::after {
    content: '';
    position: absolute;
    bottom: 0;
    left: 50%;
    width: 0;
    height: 4px;
    background-color: var(--primary-color);
    transition: all var(--transition-speed) ease;
    transform: translateX(-50%);
    border-radius: 2px 2px 0 0;
}

.nav-tab.active {
    color: var(--primary-color);
    font-weight: 600;
}

.nav-tab.active::after {
    width: 100%;
}

main {
    padding-top: calc(var(--header-height) + 2rem);
    padding-bottom: 3rem;
    max-width: 1100px; /* căn giữa */
    margin: 0 auto;
    padding-left: 2rem;
    padding-right: 2rem;
}

.page-content {
    display: none;
    animation: fadeIn 0.5s ease;
}

.page-content.active {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(15px); }
    to { opacity: 1; transform: translateY(0); }
}

.card-shadow {
    background-color: var(--card-bg);
    border-radius: var(--border-radius-md);
    box-shadow: var(--shadow-md);
    padding: 1.5rem 2rem;
    margin-bottom: 2rem;
}

.btn {
    font-family: 'Poppins', sans-serif;
    font-size: 0.95rem;
    font-weight: 600;
    padding: 0.7rem 1.2rem;
    border: none;
    border-radius: var(--border-radius-sm);
    cursor: pointer;
    transition: all var(--transition-speed) ease;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    box-shadow: var(--shadow-sm);
    transform: translateY(0);
}

.btn:hover:not(:disabled) {
    transform: translateY(-3px);
    box-shadow: var(--shadow-md);
}

.btn:active:not(:disabled) {
    transform: translateY(-1px);
    box-shadow: var(--shadow-sm);
}

.btn:disabled {
    opacity: 0.6;
    cursor: not-allowed;
    background-color: var(--border-color);
}

.btn-primary {
    background-color: var(--primary-color);
    color: var(--light-text);
}
.btn-primary:hover:not(:disabled) {
    background-color: #A93226; /* Đậm hơn */
}

.btn-secondary {
    background-color: var(--secondary-color);
    color: var(--light-text);
}
.btn-secondary:hover:not(:disabled) {
    background-color: #CA6F1E; /* Đậm hơn */
}

.btn-danger {
    background-color: var(--danger-color);
    color: var(--dark-text);
}
.btn-danger:hover:not(:disabled) {
    background-color: #7F8C8D;
}

.btn-large {
    font-size: 1.1rem;
    padding: 0.8rem 1.5rem;
    width: 100%;
    max-width: 400px;
    margin: 1rem auto;
    display: block;
}

select, input[type="search"], input[type="text"] {
    font-family: 'Poppins', sans-serif;
    font-size: 1rem;
    padding: 0.7rem 1rem;
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius-sm);
    background-color: var(--card-bg);
    min-width: 220px;
    transition: all var(--transition-speed) ease;
}

select:focus, input[type="search"]:focus, input[type="text"]:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(192, 57, 43, 0.15);
}

.controls-container, .filters-container, .flashcard-controls-container {
    display: flex;
    flex-wrap: wrap;
    gap: 1.5rem;
    align-items: flex-end; /* Căn chỉnh các item */
}

.control-group {
    display: flex;
    flex-direction: column;
    gap: 0.35rem;
}
.control-group.search-group {
    flex-grow: 1;
}
.control-group.search-group input {
    width: 100%;
}

.control-group label {
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--dark-text);
    margin-left: 0.2rem;
}

.writer-area {
    text-align: center;
}

.word-info {
    font-size: 1.2rem;
    margin-bottom: 1rem;
    color: var(--dark-text);
}
.word-info .hanzi {
    font-size: 3rem; /* To hơn */
    font-weight: 700;
    color: var(--primary-color);
    line-height: 1.2;
}
.word-info .pinyin {
    font-style: italic;
    color: var(--secondary-color);
    font-size: 1.2rem;
}
.word-info .meaning {
    font-weight: 600;
    font-size: 1.1rem;
    margin-top: 0.25rem;
}

/* Chuyển #hanzi-writer-container thành flexbox */
#hanzi-writer-container {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    margin: 1.5rem auto;
}

.char-writer-box {
    width: 220px;
    height: 220px;
    border: 2px dashed var(--border-color);
    border-radius: var(--border-radius-sm);
    background-color: #FCFCFC;
}

#hanzi-writer-container svg,
.char-writer-box svg {
    width: 100%;
    height: 100%;
}

.hanzi-controls {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 0.75rem;
}
.hanzi-controls {
    display: none; 
}

/* Khung luyện viết (Tối ưu) */
.writing-grid-container {
    margin-top: 2rem;
    border-radius: var(--border-radius-md);
    padding: 1rem;
}
.writing-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
    gap: 10px; /* Tăng khoảng cách */
    max-width: 600px;
    margin: 1rem auto;
}

.grid-cell {
    width: 100%;
    aspect-ratio: 1 / 1; /* Luôn là hình vuông */
    background-color: var(--card-bg);
    border: 1px solid var(--border-color); /* Viền nhạt hơn */
    border-radius: var(--border-radius-sm);
    box-shadow: var(--shadow-sm);
    position: relative;
    
    /* Sử dụng SVG làm nền */
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='100' height='100'%3E%3Cline x1='0' y1='0' x2='100' y2='100' stroke='%23DEE2E6' /%3E%3Cline x1='100' y1='0' x2='0' y2='100' stroke='%23DEE2E6' /%3E%3Cline x1='50' y1='0' x2='50' y2='100' stroke='%23DEE2E6' /%3E%3Cline x1='0' y1='50' x2='100' y2='50' stroke='%23DEE2E6' /%3E%3C/svg%3E");
    background-size: 100% 100%;
    background-repeat: no-repeat;
    background-position: center center;
}

/* Đảm bảo SVG của Hanzi Writer đè lên trên nền */
.grid-cell svg {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 1; /* Đảm bảo Hanzi Writer ở trên */
}

#vocab-list-container {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 1.5rem;
}

.vocab-item {
    background: var(--card-bg);
    padding: 1.25rem 1.5rem;
    border-radius: var(--border-radius-md);
    position: relative;
    transition: all var(--transition-speed) ease;
    border-left: 5px solid var(--primary-color); /* Dùng màu viền HSK */
}
.vocab-item:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-md);
}

.vocab-item .hanzi {
    font-size: 2.2rem;
    font-weight: 600;
    color: var(--primary-color);
    margin-bottom: 0.25rem;
}
.vocab-item .pinyin {
    font-style: italic;
    color: var(--secondary-color);
    font-size: 1.1rem;
}
.vocab-item .meaning {
    font-weight: 500;
    margin-top: 0.5rem;
    font-size: 1.05rem;
}
.vocab-item .level {
    font-size: 0.8rem;
    font-weight: 600;
    color: var(--light-text);
    background-color: var(--dark-text);
    padding: 0.2rem 0.6rem;
    border-radius: 20px;
    position: absolute;
    top: 1rem;
    right: 1rem;
}

.vocab-item .practice-btn {
    opacity: 0;
    visibility: hidden;
    position: absolute;
    bottom: 1.25rem;
    right: 1.5rem;
    transition: all var(--transition-speed) ease;
    transform: translateY(10px);
    padding: 0.5rem 0.8rem;
    font-size: 0.85rem;
}
.vocab-item:hover .practice-btn {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.empty-message {
    font-style: italic;
    color: #777;
    grid-column: 1 / -1; /* Chiếm toàn bộ grid */
    text-align: center;
    padding: 2rem;
}

.flashcard-controls-container {
    justify-content: center;
    align-items: flex-end;
}

.flashcard-container {
    perspective: 1500px;
    width: 100%;
    max-width: 500px;
    height: 300px;
    margin: 2.5rem auto;
}

.flashcard {
    width: 100%;
    height: 100%;
    position: relative;
    transform-style: preserve-3d;
    transition: transform 0.7s cubic-bezier(0.4, 0.2, 0.2, 1); 
    cursor: pointer;
}

.flashcard.flipped {
    transform: rotateY(180deg);
}

.flashcard-front,
.flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    -webkit-backface-visibility: hidden;
    backface-visibility: hidden;
    display: flex;
    justify-content: center;
    align-items: center;
    border-radius: var(--border-radius-md);
    box-shadow: var(--shadow-md);
    padding: 2rem;
}

.flashcard-front {
    background-color: var(--card-bg);
    color: var(--primary-color);
    font-size: 5rem;
    font-weight: 700;
    border: 1px solid var(--border-color);
}

.flashcard-back {
    /* Gradient tinh tế */
    background: linear-gradient(135deg, var(--dark-text) 0%, #4A637C 100%);
    color: var(--light-text);
    transform: rotateY(180deg);
    flex-direction: column;
    gap: 0.5rem;
    font-size: 1.2rem;
    text-align: center;
}
.flashcard-back .pinyin {
    font-size: 1.8rem;
    font-style: italic;
    color: #FFD700; /* Màu vàng gold cho pinyin */
}
.flashcard-back .meaning {
    font-size: 1.6rem;
    font-weight: 600;
    margin-top: 0.5rem;
}
.flashcard-back .level {
    font-size: 0.9rem;
    font-weight: 700;
    background: var(--light-text);
    color: var(--dark-text);
    padding: 0.3rem 0.8rem;
    border-radius: 20px;
    margin-top: 1.5rem;
}

#flashcard-progress {
    text-align: center;
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--dark-text);
}

.flashcard-nav {
    display: flex;
    justify-content: center;
    gap: 1rem;
    margin: 1.5rem 0;
}

#flashcard-practice-button {
    margin-top: 1rem;
}

.hidden-select {
    display: none; /* Ẩn select gốc */
}

.custom-select-wrapper {
    position: relative;
    width: 100%;
    min-width: 220px;
}

.custom-select-trigger {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0.7rem 1rem;
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius-sm);
    background-color: var(--card-bg);
    cursor: pointer;
    transition: all var(--transition-speed) ease;
}

.custom-select-trigger:hover {
    border-color: var(--dark-text);
}

.custom-select-wrapper.open .custom-select-trigger {
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(192, 57, 43, 0.15);
}

.custom-select-trigger span {
    font-family: 'Poppins', sans-serif;
    font-size: 1rem;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.custom-select-trigger .arrow {
    width: 0;
    height: 0;
    border-left: 5px solid transparent;
    border-right: 5px solid transparent;
    border-top: 5px solid var(--dark-text);
    transition: transform var(--transition-speed) ease;
}

.custom-select-wrapper.open .arrow {
    transform: rotate(180deg);
}

.custom-options-list {
    position: absolute;
    top: calc(100% + 5px);
    left: 0;
    right: 0;
    background-color: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius-sm);
    box-shadow: var(--shadow-md);
    z-index: 100;
    max-height: 250px;
    overflow-y: auto;
    
    /* Ẩn ban đầu */
    opacity: 0;
    visibility: hidden;
    transform: translateY(10px);
    transition: all var(--transition-speed) ease;
}

.custom-select-wrapper.open .custom-options-list {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.custom-option {
    padding: 0.8rem 1rem;
    cursor: pointer;
    font-family: 'Poppins', sans-serif;
    font-size: 1rem;
    transition: background-color 0.2s ease;
}

.custom-option:hover {
    background-color: var(--light-bg);
}

.custom-option.selected {
    background-color: var(--primary-color);
    color: var(--light-text);
    font-weight: 600;
}

@media (max-width: 768px) {
    body {
        font-size: 15px;
    }
    
    header {
        padding: 0 1rem;
    }
    .logo {
        font-size: 1.25rem;
    }
    .logo::before {
        width: 32px;
        height: 32px;
        font-size: 1.1rem;
    }
    .nav-tab {
        padding: 0 0.75rem;
        font-size: 0.9rem;
    }

    main {
        padding: calc(var(--header-height) + 1.5rem) 1rem 1.5rem;
    }
    
    .card-shadow {
        padding: 1.5rem;
    }

    .controls-container, .filters-container, .flashcard-controls-container {
        flex-direction: column;
        gap: 1rem;
        align-items: stretch; /* Kéo dãn các item */
    }
    
    .control-group select, .control-group input, .custom-select-wrapper {
        width: 100%;
        min-width: 0;
    }

    .writing-grid {
        grid-template-columns: repeat(3, 1fr);
    }
    .writing-grid-container {
        padding: 1rem;
    }

    h2 { font-size: 1.6rem; }
    h3 { font-size: 1.3rem; }
    
    /* Chỉnh kích thước ô demo cho tablet */
    .char-writer-box {
        width: 180px;
        height: 180px;
    }
    
    .flashcard-container {
        height: 250px;
    }
    .flashcard-front {
        font-size: 4rem;
    }
    .flashcard-back {
        font-size: 1rem;
    }
    .flashcard-back .pinyin {
        font-size: 1.5rem;
    }
    .flashcard-back .meaning {
        font-size: 1.3rem;
    }
}

@media (max-width: 480px) {
    header {
        flex-direction: column;
        height: auto;
        padding-bottom: 0.5rem;
    }
    .logo {
        margin: 0.75rem 0 0.5rem;
    }
    nav#main-nav {
        width: 100%;
        justify-content: space-around;
        height: 45px;
    }
    .nav-tab {
        height: 45px;
        font-size: 0.9rem;
        padding: 0 0.5rem;
        flex-grow: 1; /* Cho các tab co dãn bằng nhau */
        text-align: center;
    }

    main {
        padding-top: 130px; /* Tăng padding cho header cao hơn */
        padding-left: 0.75rem;
        padding-right: 0.75rem;
    }

    .btn {
        padding: 0.6rem 1rem;
        font-size: 0.9rem;
    }

    /* Chỉnh kích thước ô demo cho mobile */
    .char-writer-box {
        width: 150px;
        height: 150px;
    }
    
    /* Điều chỉnh lại #hanzi-writer-container cũ (nếu có) */
    #hanzi-writer-container {
        width: 100%; /* Cho phép nó co dãn */
    }

    .writing-grid {
        grid-template-columns: repeat(2, 1fr);
        gap: 10px;
    }
    
    .flashcard-nav {
        flex-wrap: wrap;
    }
    .flashcard-nav .btn {
        flex-basis: 100%;
    }
    .flashcard-nav #prev-word,
    .flashcard-nav #next-word {
        flex-basis: 48%;
    }
    .flashcard-nav #flip-card {
        order: -1; /* Đẩy nút Lật thẻ lên trên */
    }
}
footer {
    /* Căn giữa văn bản bên trong footer */
    text-align: center; 
    
    /* Các thuộc tính bổ sung để footer */
    padding: 15px 0; /* Thêm khoảng đệm trên và dưới */
    border-top: 1px solid #ccc; /* Thêm đường viền trên mỏng */
    background-color: #f8f8f8; /* Màu nền nhẹ */
    width: 100%; /* Đảm bảo chiếm hết chiều rộng */
}

footer p {
    /* Nếu bạn muốn loại bỏ lề mặc định của thẻ <p> */
    margin: 0;
    /* Căn giữa chiều ngang */
    display: inline-block; 
}