:root {
    --primary-color: #3b82f6;
    --secondary-color: #8b5cf6;
    --success-color: #10b981;
    --danger-color: #ef4444;
    --warning-color: #f59e0b;
    --dark-bg: #1f2937;
    --light-bg: #f9fafb;
    --border-color: #e5e7eb;
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    line-height: 1.6;
    color: #333;
    background-color: var(--light-bg);
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 20px;
}

/* Header */
header {
    background-color: white;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    padding: 1rem 0;
    margin-bottom: 2rem;
}

nav {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

nav h1 {
    color: var(--primary-color);
    font-size: 1.5rem;
}

nav ul {
    list-style: none;
    display: flex;
    gap: 1.5rem;
}

nav a {
    color: #666;
    text-decoration: none;
    transition: color 0.3s;
}

nav a:hover {
    color: var(--primary-color);
}

/* Buttons */
.btn {
    padding: 0.75rem 1.5rem;
    border: none;
    border-radius: 0.5rem;
    cursor: pointer;
    font-size: 1rem;
    transition: all 0.3s;
    text-decoration: none;
    display: inline-block;
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
}

.btn-primary:hover {
    background-color: #2563eb;
}

.btn-secondary {
    background-color: var(--secondary-color);
    color: white;
}

.btn-success {
    background-color: var(--success-color);
    color: white;
}

.btn-danger {
    background-color: var(--danger-color);
    color: white;
}

/* Forms */
.form-group {
    margin-bottom: 1.5rem;
}

label {
    display: block;
    margin-bottom: 0.5rem;
    font-weight: 500;
}

input, textarea, select {
    width: 100%;
    padding: 0.75rem;
    border: 1px solid var(--border-color);
    border-radius: 0.5rem;
    font-size: 1rem;
}

textarea {
    min-height: 150px;
    resize: vertical;
}

/* Cards */
.card {
    background-color: white;
    border-radius: 0.5rem;
    padding: 1.5rem;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    margin-bottom: 1.5rem;
}

.card-header {
    font-size: 1.25rem;
    font-weight: 600;
    margin-bottom: 1rem;
    color: var(--dark-bg);
}

/* Flashcard */
.flashcard {
    perspective: 1000px;
    min-height: 400px;
    margin: 2rem auto;
    max-width: 600px;
}

.flashcard-inner {
    position: relative;
    width: 100%;
    height: 400px;
    transition: transform 0.6s;
    transform-style: preserve-3d;
}

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

.flashcard-front, .flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    padding: 2rem;
    background: white;
    border-radius: 1rem;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}

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

.word {
    font-size: 3rem;
    font-weight: bold;
    color: var(--primary-color);
    margin-bottom: 1rem;
}

.phonetic {
    font-size: 1.5rem;
    color: #666;
    margin-bottom: 2rem;
}

.definition {
    font-size: 1.25rem;
    text-align: center;
    margin-bottom: 1rem;
}

/* Alert */
.alert {
    padding: 1rem;
    border-radius: 0.5rem;
    margin-bottom: 1rem;
}

.alert-success {
    background-color: #d1fae5;
    color: #065f46;
}

.alert-error {
    background-color: #fee2e2;
    color: #991b1b;
}

.alert-info {
    background-color: #dbeafe;
    color: #1e40af;
}

/* Loading */
.loading {
    text-align: center;
    padding: 2rem;
}

.spinner {
    border: 4px solid #f3f4f6;
    border-top: 4px solid var(--primary-color);
    border-radius: 50%;
    width: 40px;
    height: 40px;
    animation: spin 1s linear infinite;
    margin: 0 auto;
}

@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}

/* Utility classes */
.text-center {
    text-align: center;
}

.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mt-3 { margin-top: 1.5rem; }
.mt-4 { margin-top: 2rem; }

.mb-1 { margin-bottom: 0.5rem; }
.mb-2 { margin-bottom: 1rem; }
.mb-3 { margin-bottom: 1.5rem; }
.mb-4 { margin-bottom: 2rem; }

.hidden {
    display: none;
}