:root {
--color-primary: #1E3A8A;
--color-primary-light: #2563EB;
--color-primary-dark: #1D4ED8;
--color-background: #F8FAFC;
--color-text: #1E293B;
--color-text-light: #64748B;
--color-white: #FFFFFF;
--color-border: #E2E8F0;

--shadow-sm: 0 2px 4px rgba(0,0,0,0.05);
--shadow-md: 0 4px 12px rgba(30, 58, 138, 0.08);
--shadow-lg: 0 8px 24px rgba(30, 58, 138, 0.12);

--spacing-xs: 0.5rem;
--spacing-sm: 1rem;
--spacing-md: 1.5rem;
--spacing-lg: 2rem;
--spacing-xl: 2.5rem;

--border-radius-sm: 8px;
--border-radius-md: 12px;
--border-radius-lg: 16px;

--transition-fast: 0.2s ease;
--transition-normal: 0.3s ease;
}

@media (prefers-reduced-motion: reduce) {
* {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
}
}

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

html {
scroll-behavior: smooth;
}

body {
background-color: var(--color-background);
min-height: 100vh;
display: flex;
flex-direction: column;
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
color: var(--color-text);
line-height: 1.5;
}

.skip-link {
position: absolute;
top: -40px;
left: 0;
background: var(--color-primary);
color: var(--color-white);
padding: var(--spacing-sm) var(--spacing-md);
z-index: 100;
transition: top var(--transition-normal);
}

.skip-link:focus {
top: 0;
outline: 2px solid var(--color-primary-light);
outline-offset: 2px;
}

.navbar {
background: var(--color-primary);
padding: var(--spacing-md) var(--spacing-xl);
color: var(--color-white);
display: flex;
justify-content: space-between;
align-items: center;
box-shadow: var(--shadow-md);
position: relative;
z-index: 10;
}

.logo {
font-size: 1.75rem;
font-weight: bold;
color: var(--color-white);
text-decoration: none;
transition: transform var(--transition-normal);
}

.logo:hover,
.logo:focus {
transform: translateY(-2px);
}

.nav-links a {
color: var(--color-white);
text-decoration: none;
margin-left: var(--spacing-md);
font-weight: 500;
transition: all var(--transition-normal);
padding: var(--spacing-xs) var(--spacing-sm);
border-radius: var(--border-radius-sm);
}

.nav-links a:hover,
.nav-links a:focus {
background: rgba(255, 255, 255, 0.1);
outline: none;
}

.nav-links a[aria-current="page"] {
background: rgba(255, 255, 255, 0.15);
}

.hero-section {
background: linear-gradient(135deg, var(--color-primary) 0%, var(--color-primary-light) 100%);
padding: 4rem var(--spacing-lg) 6rem;
text-align: center;
color: var(--color-white);
position: relative;
overflow: hidden;
margin-bottom: 0;
}

.hero-section::before {
content: '';
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
background: radial-gradient(circle at 75% 25%, rgba(255,255,255,0.1) 0%, transparent 60%);
z-index: 1;
}

.hero-title {
font-size: 3.5rem;
font-weight: 800;
margin-bottom: 1.5rem;
position: relative;
z-index: 2;
line-height: 1.2;
letter-spacing: -0.5px;
}

.hero-description {
font-size: 1.25rem;
max-width: 700px;
margin: 0 auto 2rem;
opacity: 0.9;
position: relative;
z-index: 2;
line-height: 1.6;
}

.container {
max-width: 1400px;
margin: 0 auto;
padding: 4rem 2rem;
position: relative;
min-height: 900px;
box-sizing: border-box;
}

.main-categories {
display: flex;
justify-content: space-between;
gap: 1.5rem;
margin-bottom: 3rem;
width: 100%;
box-sizing: border-box;
padding: 0;
background-color: transparent;
}

.main-category {
flex: 1;
padding: 1.75rem;
background: none;
border-radius: 16px;
text-align: center;
cursor: pointer;
transition: all 0.3s ease;
border: none;
box-shadow: 0 4px 12px rgba(30, 58, 138, 0.08);
color: var(--color-white);
}

.main-category[data-category='hot'] {
background: #FF5722; /* 橙色 */
}

.main-category[data-category='video'] {
background: #2196F3; /* 蓝色 */
}

.main-category[data-category='writing'] {
background: #4CAF50; /* 绿色 */
}

.main-category[data-category='expert'] {
background: #FF9800; /* 深橙色 */
}

.main-category.active {
border-color: var(--color-primary);
box-shadow: var(--shadow-lg);
}

.main-category:focus {
outline: none;
box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.3);
}

.main-category:hover {
transform: translateY(-5px);
box-shadow: 0 8px 20px rgba(30, 58, 138, 0.15);
background-color: var(--color-primary-dark);
}

.main-category h3 {
color: white;
font-size: 1.3rem;
margin: 0;
text-shadow: 0 1px 2px rgba(0,0,0,0.1);
}

.category-grid {
display: grid;
grid-template-columns: repeat(4, 1fr);
gap: 2rem;
margin-bottom: 3rem;
padding: 2rem;
background: rgba(255, 255, 255, 0.95);
border-radius: 24px;
border: 1px solid rgba(30, 58, 138, 0.1);
opacity: 0;
visibility: hidden;
transition: opacity 0.3s ease, visibility 0.3s ease, min-height 0.3s ease;
position: absolute;
width: 100%;
min-height: 800px;
left: 0;
top: 7rem;
box-sizing: border-box;
}

.category-grid.active {
opacity: 1;
visibility: visible;
position: relative;
left: 0;
top: 0;
}

.category-card {
background: white;
border-radius: 16px;
padding: 2.5rem;
box-shadow: 0 4px 12px rgba(30, 58, 138, 0.08);
transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
display: flex;
flex-direction: column;
text-decoration: none;
color: inherit;
position: relative;
overflow: hidden;
border: 1px solid rgba(30, 58, 138, 0.05);
}

.category-card::before {
content: '';
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
background: linear-gradient(135deg, rgba(30, 58, 138, 0.02) 0%, rgba(59, 130, 246, 0.02) 100%);
z-index: -1;
border-radius: 16px;
}

.category-card:hover {
transform: translateY(-5px);
box-shadow: 0 8px 24px rgba(30, 58, 138, 0.12);
}

.category-number {
font-size: 1rem;
color: #3B82F6;
margin-bottom: 1.5rem;
font-weight: 600;
display: inline-block;
padding: 0.5rem 1rem;
background: rgba(59, 130, 246, 0.1);
border-radius: 8px;
}

.category-title {
font-size: 1.6rem;
color: #1E3A8A;
margin-bottom: 1.5rem;
font-weight: 700;
line-height: 1.4;
display: flex;
align-items: center;
gap: 1rem;
}

.category-description {
color: #64748B;
font-size: 1.15rem;
line-height: 1.6;
margin-bottom: 2.5rem;
flex-grow: 1;
}

.category-button {
display: inline-block;
padding: var(--spacing-sm) var(--spacing-lg);
background: var(--color-primary-light);
color: var(--color-white);
text-decoration: none;
border-radius: var(--border-radius-sm);
font-weight: 600;
transition: all var(--transition-normal);
text-align: center;
width: 100%;
border: 2px solid transparent;
}

.category-button:hover,
.category-button:focus {
background: var(--color-primary-dark);
transform: translateY(-2px);
box-shadow: var(--shadow-md);
}

.category-button:focus {
outline: none;
border-color: var(--color-primary);
}

footer {
text-align: center;
padding: var(--spacing-xl);
color: var(--color-text-light);
background: var(--color-white);
margin-top: auto;
border-top: 1px solid var(--color-border);
}

.footer-links {
margin-top: var(--spacing-sm);
display: flex;
justify-content: center;
gap: var(--spacing-md);
align-items: center;
}

.footer-links a {
color: var(--color-text-light);
text-decoration: none;
transition: color var(--transition-normal);
}

.footer-links a:hover,
.footer-links a:focus {
color: var(--color-primary);
}

.language-hint {
color: var(--color-text-light);
font-size: 0.9rem;
}

@media (max-width: 1200px) {
.category-grid {
    grid-template-columns: repeat(3, 1fr);
}
}

@media (max-width: 992px) {
.hero-title {
    font-size: 3rem;
}

.hero-description {
    font-size: 1.3rem;
}

.category-grid {
    grid-template-columns: repeat(2, 1fr);
}

.main-categories {
    flex-wrap: wrap;
}

.main-category {
    flex: 1 1 calc(50% - 0.75rem);
}
}

@media (max-width: 768px) {
.hero-title {
    font-size: 2.5rem;
}

.hero-description {
    font-size: 1.2rem;
}

.navbar {
    padding: 1.25rem 2rem;
}

.logo {
    font-size: 1.5rem;
}
}

@media (max-width: 576px) {
.hero-title {
    font-size: 2rem;
    padding: 0 1rem;
}

.hero-description {
    font-size: 1.1rem;
    padding: 0 1rem;
}

.category-grid {
    grid-template-columns: 1fr;
    padding: 1rem;
    gap: 1rem;
    width: 100%;
}

.main-category {
    flex: 1 1 100%;
    padding: 1.25rem;
}

.main-categories {
    width: 100%;
    padding: 0;
}

.category-card {
    padding: 1.5rem;
}

.container {
    padding: 0 1rem;
    margin: 1.5rem auto;
}
}

/* Formula Page Styles */
.header {
text-align: center;
margin-bottom: var(--spacing-xl);
padding: var(--spacing-lg) var(--spacing-sm);
background: var(--color-white);
border-radius: var(--border-radius-md);
box-shadow: var(--shadow-sm);
}

.header h1 {
color: var(--color-primary);
font-size: 2.25rem;
margin-bottom: var(--spacing-sm);
line-height: 1.2;
}

.header p {
color: var(--color-text-light);
font-size: 1.1rem;
max-width: 600px;
margin: 0 auto;
line-height: 1.6;
}

.formula-list {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: 1.5rem;
padding: 1rem 0;
max-width: 1200px;
margin: 0 auto;
}

.formula-card {
background: var(--color-white);
border-radius: var(--border-radius-md);
padding: 1.75rem;
box-shadow: var(--shadow-sm);
transition: all var(--transition-normal);
cursor: pointer;
height: 100%;
display: flex;
flex-direction: column;
position: relative;
overflow: hidden;
text-decoration: none;
color: inherit;
border: 1px solid var(--color-border);
}

.formula-card:hover,
.formula-card:focus {
transform: translateY(-3px);
box-shadow: var(--shadow-lg);
outline: none;
border-color: var(--color-primary-light);
}

.formula-card:focus-visible {
box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.3);
}

.formula-number {
font-size: 0.9rem;
color: var(--color-primary-light);
margin-bottom: var(--spacing-sm);
font-weight: 500;
display: inline-block;
padding: calc(var(--spacing-xs) / 2) var(--spacing-sm);
background: rgba(59, 130, 246, 0.1);
border-radius: calc(var(--border-radius-sm) / 2);
}

.formula-title {
font-size: 1.25rem;
color: var(--color-primary);
margin-bottom: var(--spacing-sm);
font-weight: 600;
line-height: 1.4;
}

.formula-description {
color: var(--color-text-light);
font-size: 0.95rem;
line-height: 1.6;
flex-grow: 1;
margin-bottom: var(--spacing-md);
}

.formula-buttons {
display: flex;
gap: 10px;
margin-top: auto;
padding-top: 1rem;
}

.formula-button {
text-decoration: none;
padding: calc(var(--spacing-sm) - 4px) var(--spacing-md);
background: var(--color-primary-light);
color: var(--color-white);
border-radius: calc(var(--border-radius-sm) - 2px);
font-size: 0.9rem;
font-weight: 600;
transition: all var(--transition-normal);
border: 2px solid transparent;
text-align: center;
flex: 1;
}

.formula-button:hover,
.formula-button:focus {
background: var(--color-primary-dark);
transform: translateY(-2px);
box-shadow: var(--shadow-sm);
outline: none;
}

.formula-button:focus-visible {
border-color: var(--color-primary);
box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.3);
}

.formula-button.secondary {
background: var(--color-writing);
color: var(--color-white);
}

.formula-button.secondary:hover,
.formula-button.secondary:focus {
background: #047857;
}

@media (max-width: 1024px) {
.formula-list {
    grid-template-columns: repeat(2, 1fr);
    padding: 1rem;
}
}

@media (max-width: 640px) {
.formula-list {
    grid-template-columns: 1fr;
}

.container {
    padding: 1rem;
}

.formula-card {
    padding: 1.25rem;
}

.header {
    padding: 1.5rem 1rem;
    margin-bottom: 2rem;
}

.header h1 {
    font-size: 1.75rem;
}
}

/* 确保分类内容区域与导航对齐 */
.category-grid-container {
width: 100%;
position: relative;
}