:root {
    --bg-gradient-start: #0ea5e9;
    --bg-gradient-end: #06b6d4;
    --card-gradient-start: #f59e0b;
    --card-gradient-end: #f97316;
    --card-back: #ffffff;
    --text-primary: #1f2937;
    --text-secondary: #6b7280;
    --text-on-gradient: #ffffff;
    --accent-primary: #0ea5e9;
    --accent-secondary: #06b6d4;
    --example-bg: #f3f4f6;
    --tip-bg: #e0f2fe;
    --shadow: rgba(0, 0, 0, 0.1);
    --shadow-hover: rgba(0, 0, 0, 0.15);
    --overlay-light: rgba(255, 255, 255, 0.15);
    --overlay-lighter: rgba(255, 255, 255, 0.25);
}

body.dark-mode {
    --bg-gradient-start: #1e1b4b;
    --bg-gradient-end: #312e81;
    --card-gradient-start: #be185d;
    --card-gradient-end: #9f1239;
    --card-back: #1f2937;
    --text-primary: #f9fafb;
    --text-secondary: #d1d5db;
    --text-on-gradient: #ffffff;
    --accent-primary: #818cf8;
    --accent-secondary: #a78bfa;
    --example-bg: #374151;
    --tip-bg: #312e81;
    --shadow: rgba(0, 0, 0, 0.3);
    --shadow-hover: rgba(0, 0, 0, 0.4);
    --overlay-light: rgba(255, 255, 255, 0.1);
    --overlay-lighter: rgba(255, 255, 255, 0.2);
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Cantarell', sans-serif;
    background: linear-gradient(135deg, var(--bg-gradient-start) 0%, var(--bg-gradient-end) 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
    color: var(--text-primary);
    transition: background 0.3s ease;
}

.theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
    width: 60px;
    height: 60px;
    border-radius: 50%;
    border: none;
    background: var(--overlay-light);
    backdrop-filter: blur(10px);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.8rem;
    transition: all 0.3s ease;
    box-shadow: 0 4px 15px var(--shadow);
    z-index: 1000;
}

.theme-toggle:hover {
    background: var(--overlay-lighter);
    transform: scale(1.1) rotate(20deg);
}

.theme-toggle:active {
    transform: scale(0.95);
}

.sun-icon,
.moon-icon {
    position: absolute;
    transition: all 0.3s ease;
}

.sun-icon {
    opacity: 1;
    transform: rotate(0deg) scale(1);
}

.moon-icon {
    opacity: 0;
    transform: rotate(180deg) scale(0);
}

body.dark-mode .sun-icon {
    opacity: 0;
    transform: rotate(-180deg) scale(0);
}

body.dark-mode .moon-icon {
    opacity: 1;
    transform: rotate(0deg) scale(1);
}

.lang-toggle {
    position: fixed;
    top: 20px;
    left: 20px;
    padding: 10px 20px;
    border-radius: 25px;
    border: none;
    background: var(--overlay-light);
    backdrop-filter: blur(10px);
    cursor: pointer;
    font-size: 1rem;
    font-weight: 700;
    color: var(--text-on-gradient);
    transition: all 0.3s ease;
    box-shadow: 0 4px 15px var(--shadow);
    z-index: 1000;
    letter-spacing: 1px;
}

.lang-toggle:hover {
    background: var(--overlay-lighter);
    transform: scale(1.05);
}

.lang-toggle:active {
    transform: scale(0.95);
}

.container {
    max-width: 600px;
    width: 100%;
}

header {
    text-align: center;
    margin-bottom: 40px;
    color: var(--text-on-gradient);
}

header h1 {
    font-size: 2.5rem;
    margin-bottom: 10px;
    text-shadow: 2px 2px 4px var(--shadow);
    font-weight: 700;
    letter-spacing: -0.5px;
}

.subtitle {
    font-size: 1.1rem;
    opacity: 0.95;
    font-weight: 400;
}

.card-deck {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
    padding: 0 10px;
}

.deck-indicator {
    background: var(--overlay-light);
    padding: 10px 18px;
    border-radius: 25px;
    color: var(--text-on-gradient);
    font-weight: 600;
    backdrop-filter: blur(10px);
    border: 1px solid var(--overlay-lighter);
    font-size: 0.95rem;
}

.shuffle-btn {
    background: var(--overlay-light);
    border: 1px solid var(--overlay-lighter);
    color: var(--text-on-gradient);
    padding: 10px 22px;
    border-radius: 25px;
    cursor: pointer;
    font-weight: 600;
    transition: all 0.3s ease;
    backdrop-filter: blur(10px);
    font-size: 0.95rem;
}

.shuffle-btn:hover {
    background: var(--overlay-lighter);
    transform: scale(1.05);
}

.shuffle-btn:active {
    transform: scale(0.98);
}

.card-container {
    perspective: 1000px;
    margin-bottom: 30px;
}

.card {
    width: 100%;
    height: 650px;
    cursor: pointer;
    position: relative;
}

.card-inner {
    width: 100%;
    height: 100%;
    transition: transform 0.6s;
    transform-style: preserve-3d;
    position: relative;
}

.card.flipped .card-inner {
    transform: rotateY(180deg);
}

.card-front,
.card-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    border-radius: 24px;
    padding: 40px;
    box-shadow: 0 20px 60px var(--shadow-hover);
}

.card-front {
    background: linear-gradient(135deg, var(--card-gradient-start) 0%, var(--card-gradient-end) 100%);
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    color: var(--text-on-gradient);
}

.card-icon {
    font-size: 5rem;
    margin-bottom: 20px;
    animation: float 3s ease-in-out infinite;
    filter: drop-shadow(0 4px 8px rgba(0, 0, 0, 0.1));
}

@keyframes float {
    0%, 100% {
        transform: translateY(0px);
    }
    50% {
        transform: translateY(-20px);
    }
}

.card-title {
    font-size: 2rem;
    margin-bottom: 10px;
    font-weight: 700;
}

.card-hint {
    font-size: 1.1rem;
    opacity: 0.95;
    font-weight: 400;
}

.card-back {
    background: var(--card-back);
    transform: rotateY(180deg);
    overflow-y: auto;
    border: 1px solid var(--overlay-light);
}

.card-back::-webkit-scrollbar {
    width: 8px;
}

.card-back::-webkit-scrollbar-track {
    background: transparent;
}

.card-back::-webkit-scrollbar-thumb {
    background: var(--accent-primary);
    border-radius: 4px;
}

.card-back::-webkit-scrollbar-thumb:hover {
    background: var(--accent-secondary);
}

.concept-header {
    display: flex;
    align-items: center;
    gap: 15px;
    margin-bottom: 25px;
    padding-bottom: 20px;
    border-bottom: 3px solid var(--accent-primary);
}

.concept-icon {
    font-size: 3rem;
    filter: drop-shadow(0 2px 4px var(--shadow));
}

.concept-header h2 {
    color: var(--accent-primary);
    font-size: 1.8rem;
    font-weight: 700;
}

.concept-content {
    line-height: 1.7;
}

.concept-description {
    font-size: 1.1rem;
    margin-bottom: 20px;
    color: var(--text-secondary);
    font-weight: 400;
}

.concept-example {
    background: var(--example-bg);
    padding: 16px;
    border-radius: 12px;
    margin-bottom: 20px;
    border-left: 4px solid var(--accent-primary);
}

.concept-example strong {
    color: var(--accent-primary);
    display: block;
    margin-bottom: 8px;
    font-weight: 600;
}

.concept-tip {
    background: var(--tip-bg);
    padding: 16px;
    border-radius: 12px;
    font-style: italic;
    border: 1px solid var(--accent-secondary);
}

.concept-tip strong {
    color: var(--accent-primary);
    font-style: normal;
    display: block;
    margin-bottom: 8px;
    font-weight: 600;
}

.controls {
    display: flex;
    gap: 15px;
    justify-content: center;
    margin-bottom: 30px;
}

.nav-btn {
    padding: 14px 32px;
    border: none;
    border-radius: 30px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s ease;
    background: var(--overlay-light);
    color: var(--text-on-gradient);
    box-shadow: 0 4px 15px var(--shadow);
    backdrop-filter: blur(10px);
    border: 1px solid var(--overlay-lighter);
}

.nav-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px var(--shadow-hover);
    background: var(--overlay-lighter);
}

.nav-btn:active {
    transform: translateY(0);
}

.nav-btn.primary {
    background: linear-gradient(135deg, var(--accent-primary) 0%, var(--accent-secondary) 100%);
    color: white;
    border: none;
}

.progress-dots {
    display: flex;
    justify-content: center;
    gap: 10px;
    flex-wrap: wrap;
    padding: 0 20px;
}

.progress-dot {
    width: 10px;
    height: 10px;
    border-radius: 50%;
    background: var(--overlay-light);
    transition: all 0.3s ease;
    cursor: pointer;
    border: 1px solid var(--overlay-lighter);
}

.progress-dot.active {
    background: var(--text-on-gradient);
    transform: scale(1.4);
    box-shadow: 0 2px 8px var(--shadow);
}

.progress-dot:hover {
    background: var(--overlay-lighter);
    transform: scale(1.2);
}

@media (max-width: 600px) {
    .theme-toggle {
        width: 50px;
        height: 50px;
        font-size: 1.5rem;
        top: 15px;
        right: 15px;
    }

    .lang-toggle {
        padding: 8px 16px;
        font-size: 0.9rem;
        top: 15px;
        left: 15px;
    }

    header h1 {
        font-size: 2rem;
    }

    .subtitle {
        font-size: 1rem;
    }

    .card {
        height: 600px;
    }

    .card-front,
    .card-back {
        padding: 30px;
    }

    .concept-header h2 {
        font-size: 1.5rem;
    }

    .concept-description {
        font-size: 1rem;
    }

    .nav-btn {
        padding: 12px 24px;
        font-size: 0.9rem;
    }
}