/* Global Styles */
:root {
    /* Color Palette - Premium & Modern */
    --primary-color: #2563eb;
    /* Vibrant Blue */
    --primary-hover: #1d4ed8;
    --secondary-color: #64748b;
    --bg-color: #f8fafc;
    /* Cool Gray Background */
    --surface-color: #ffffff;
    --text-main: #0f172a;
    /* Slate 900 */
    --text-secondary: #475569;
    /* Slate 600 */
    --text-muted: #94a3b8;
    /* Slate 400 */
    --border-color: #e2e8f0;
    /* Slate 200 */

    /* Status Colors */
    --success-bg: #ecfdf5;
    --success-text: #059669;
    --success-border: #a7f3d0;
    --error-bg: #fef2f2;
    --error-text: #dc2626;
    --error-border: #fecaca;
    --warning-bg: #fffbeb;
    --warning-text: #d97706;
    --warning-border: #fde68a;

    /* Shadows & Effects */
    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
    --radius-sm: 0.375rem;
    --radius-md: 0.5rem;
    --radius-lg: 0.75rem;
    --transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
    background: var(--bg-color);
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    color: var(--text-main);
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

.container {
    max-width: 1400px;
    margin: 0 auto;
    padding: 3rem 2rem;
}

/* Header with Glassmorphism feel if we had a background image, but here clean & minimal */
.header {
    text-align: center;
    margin-bottom: 4rem;
    padding-bottom: 2.5rem;
    border-bottom: 1px solid var(--border-color);
    position: relative;
}

.header::after {
    content: '';
    position: absolute;
    bottom: -1px;
    left: 50%;
    transform: translateX(-50%);
    width: 60px;
    height: 3px;
    background: var(--primary-color);
    border-radius: 2px;
}

.header h1 {
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--text-main);
    margin-bottom: 0.75rem;
    letter-spacing: -0.03em;
    background: linear-gradient(135deg, #1e293b 0%, #334155 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

.header p {
    color: var(--text-secondary);
    font-size: 1.125rem;
    font-weight: 400;
}

.content {
    max-width: 1200px;
    margin: 0 auto;
}

/* Cards & Sections */
.form-section,
.card {
    background: var(--surface-color);
    border-radius: var(--radius-lg);
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}

.form-section {
    margin-bottom: 2rem;
    padding: 2rem;
}

.form-section:hover,
.card:hover {
    box-shadow: var(--shadow-md);
    border-color: #cbd5e1;
}

.card-body {
    padding: 2rem;
}

/* Forms */
.form-label {
    display: block;
    font-weight: 600;
    color: var(--text-main);
    margin-bottom: 0.75rem;
    font-size: 0.95rem;
}

.form-control {
    width: 100%;
    padding: 0.875rem 1rem;
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    font-size: 1rem;
    background: var(--surface-color);
    transition: var(--transition);
    color: var(--text-main);
}

.form-control:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 4px rgba(37, 99, 235, 0.1);
}

.form-control::placeholder {
    color: var(--text-muted);
}

/* Buttons */
.btn {
    padding: 0.875rem 1.75rem;
    border: none;
    border-radius: var(--radius-md);
    font-size: 0.95rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
    text-decoration: none;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    letter-spacing: 0.01em;
}

.btn-primary {
    background: var(--primary-color);
    color: white;
    box-shadow: 0 1px 2px rgba(37, 99, 235, 0.2);
}

.btn-primary:hover {
    background: var(--primary-hover);
    transform: translateY(-1px);
    box-shadow: 0 4px 6px -1px rgba(37, 99, 235, 0.3);
}

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

.btn-outline {
    background: transparent;
    color: var(--text-secondary);
    border: 1px solid var(--border-color);
}

.btn-outline:hover {
    background: #f1f5f9;
    border-color: #cbd5e1;
    color: var(--text-main);
    transform: translateY(-1px);
}

.btn-block {
    width: 100%;
}

/* Assignment Cards */
.assignment-card {
    margin-bottom: 1rem;
    cursor: pointer;
    transition: var(--transition);
    border: 1px solid var(--border-color);
    background: var(--surface-color);
    border-radius: var(--radius-lg);
    overflow: hidden;
}

.assignment-card:hover {
    border-color: var(--primary-color);
    box-shadow: var(--shadow-lg);
    transform: translateY(-4px);
}

.assignment-title {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-main);
    margin-bottom: 0.5rem;
    letter-spacing: -0.02em;
}

.assignment-meta {
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.5;
}

/* Student List Items */
.student-item {
    padding: 1rem 1.25rem;
    border-bottom: 1px solid var(--bg-color);
    cursor: pointer;
    transition: var(--transition);
    font-weight: 500;
    color: var(--text-secondary);
    display: flex;
    align-items: center;
}

.student-item:hover {
    background: #f1f5f9;
    color: var(--primary-color);
    padding-left: 1.5rem;
}

.student-item:last-child {
    border-bottom: none;
}

/* File Upload Area */
.file-upload-area {
    border: 2px dashed var(--border-color);
    border-radius: var(--radius-lg);
    padding: 4rem 2rem;
    text-align: center;
    cursor: pointer;
    transition: var(--transition);
    background: #f8fafc;
}

.file-upload-area:hover,
.file-upload-area.dragover {
    border-color: var(--primary-color);
    background: #eff6ff;
    transform: scale(1.01);
}

/* Alerts & Toasts */
.alert {
    padding: 1rem 1.25rem;
    border-radius: var(--radius-md);
    margin-bottom: 1.5rem;
    border: 1px solid transparent;
    font-weight: 500;
    display: flex;
    align-items: center;
    gap: 0.75rem;
}

.alert-success {
    background: var(--success-bg);
    color: var(--success-text);
    border-color: var(--success-border);
}

.alert-error {
    background: var(--error-bg);
    color: var(--error-text);
    border-color: var(--error-border);
}

/* Progress Bar */
.progress {
    width: 100%;
    height: 8px;
    background: var(--border-color);
    border-radius: 4px;
    overflow: hidden;
}

.progress-bar {
    height: 100%;
    background: var(--primary-color);
    transition: width 0.3s ease;
    border-radius: 4px;
}

/* Utilities */
.text-center {
    text-align: center;
}

.text-muted {
    color: var(--text-muted);
}

.mt-1 {
    margin-top: 0.25rem;
}

.mt-2 {
    margin-top: 0.5rem;
}

.mt-3 {
    margin-top: 1rem;
}

.mb-1 {
    margin-bottom: 0.25rem;
}

.mb-2 {
    margin-bottom: 0.5rem;
}

.mb-3 {
    margin-bottom: 1rem;
}

.d-none {
    display: none;
}

.d-block {
    display: block;
}

/* Responsive */
@media (max-width: 1024px) {
    .container {
        padding: 2rem 1.5rem;
    }
}

@media (max-width: 768px) {
    .container {
        padding: 1.5rem 1rem;
    }

    .header h1 {
        font-size: 2rem;
    }

    .form-section {
        padding: 1.5rem;
    }
}