/* mobile-first */
:root {
  --primary-color: #1e73be;
  --secondary-color: #0a3d62;
  --background-color: #f5f7fa;
  --white: #ffffff;
  --text-main: #333333;
  --text-muted: #666666;
  --border-color: #e0e0e0;
  --danger: #e74c3c;
  --success: #27ae60;
  --radius-md: 8px;
  --radius-lg: 12px;
  --shadow-sm: 0 2px 4px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 6px rgba(0,0,0,0.1);
  --font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
}
* { box-sizing: border-box; margin: 0; padding: 0; }
body { font-family: var(--font-family); background-color: var(--background-color); color: var(--text-main); line-height: 1.6; }
a { text-decoration: none; color: var(--primary-color); }
a:hover { text-decoration: underline; }

.container { width: 100%; max-width: 1200px; margin: 0 auto; padding: 0 15px; }

/* Header */
.top-header { background-color: var(--secondary-color); color: var(--white); font-size: 0.85rem; padding: 5px 0; text-align: center; }
.main-header { background-color: var(--white); box-shadow: var(--shadow-sm); position: sticky; top: 0; z-index: 1000; }
.header-container { display: flex; justify-content: space-between; align-items: center; padding: 15px; max-width: 1200px; margin: 0 auto; }
.logo a { font-size: 1.5rem; font-weight: 700; color: var(--secondary-color); text-decoration: none; }
.nav-links { display: none; gap: 20px; }
.nav-links a { color: var(--text-main); font-weight: 500; text-decoration: none; padding: 5px 0; }
.nav-links a:hover, .nav-links a.active { color: var(--primary-color); }
.mobile-menu-btn { display: block; background: none; border: none; font-size: 1.5rem; cursor: pointer; color: var(--secondary-color); }
@media(min-width: 768px) { .nav-links { display: flex; } .mobile-menu-btn { display: none; } }
.nav-links.show { display: flex; flex-direction: column; position: absolute; top: 100%; left: 0; right: 0; background: var(--white); padding: 20px; box-shadow: var(--shadow-md); }

/* Buttons & Grids */
.btn { display: inline-block; padding: 10px 20px; background-color: var(--primary-color); color: var(--white); border-radius: var(--radius-md); text-align: center; border: none; cursor: pointer; font-weight: 600; }
.btn:hover { background-color: var(--secondary-color); color: var(--white); }
.btn-block { display: block; width: 100%; }

.grid { display: grid; grid-template-columns: 1fr; gap: 20px; margin-top: 20px; }
@media(min-width: 600px) { .grid { grid-template-columns: repeat(2, 1fr); } }
@media(min-width: 992px) { .grid { grid-template-columns: repeat(3, 1fr); } }

.grid-2 { display: grid; grid-template-columns: 1fr; gap: 20px; }
@media(min-width: 768px) { .grid-2 { grid-template-columns: repeat(2, 1fr); } }

.grid-4 { display: grid; grid-template-columns: 1fr; gap: 20px; }
@media(min-width: 600px) { .grid-4 { grid-template-columns: repeat(2, 1fr); } }
@media(min-width: 992px) { .grid-4 { grid-template-columns: repeat(4, 1fr); } }

.card { background: var(--white); border-radius: var(--radius-md); overflow: hidden; box-shadow: var(--shadow-sm); transition: transform 0.2s; display: flex; flex-direction: column; }
.card:hover { transform: translateY(-5px); box-shadow: var(--shadow-md); }
.card-img { height: 200px; width: 100%; object-fit: cover; background-color: #ddd; }
.card-body { padding: 20px; flex-grow: 1; display:flex; flex-direction: column; }
.card-title { font-size: 1.25rem; margin-bottom: 10px; color: var(--secondary-color); }
.card-text { color: var(--text-muted); margin-bottom: 15px; flex-grow: 1; }

.main-content { padding: 40px 0; }
.section-title { text-align: center; margin-bottom: 30px; color: var(--secondary-color); font-size: 2rem; }

.hero { text-align: center; padding: 60px 20px; background: linear-gradient(135deg, var(--secondary-color), var(--primary-color)); color: var(--white); border-radius: var(--radius-lg); margin-bottom: 40px; }
.hero h1 { font-size: 2.5rem; margin-bottom: 20px; }
.hero p { font-size: 1.25rem; margin-bottom: 30px; opacity: 0.9; }

.ad-slot { background-color: #e9ecef; margin: 30px auto; display: flex; align-items: center; justify-content: center; color: #adb5bd; text-align: center; border: 1px solid #ced4da; border-radius: 4px; }
.ad-leaderboard { width: 100%; max-width: 728px; height: 90px; }
.ad-responsive { width: 100%; min-height: 250px; }

/* Content Items */
.article-header { margin-bottom: 30px; }
.article-title { font-size: 2.2rem; color: var(--secondary-color); margin-bottom: 10px; }
.article-meta { color: var(--text-muted); font-size: 0.9rem; margin-bottom: 20px; }
.article-content { font-size: 1.1rem; }
.article-content h2 { color: var(--secondary-color); margin: 35px 0 15px; font-size: 1.8rem; }
.article-content h3 { color: var(--primary-color); margin: 25px 0 10px; font-size: 1.4rem; }
.article-content p { margin-bottom: 20px; }
.article-content ul, .article-content ol { margin-bottom: 20px; padding-left: 20px; }
.article-content li { margin-bottom: 10px; }

.toc { background: #f8fbff; padding: 25px; border-radius: var(--radius-md); border: 1px solid #cce5ff; margin-bottom: 35px; }
.toc h3 { margin-top: 0; margin-bottom: 15px; color: var(--secondary-color); }
.toc ul { list-style: none; padding-left: 0; margin-bottom: 0; }
.toc li { margin-bottom: 10px; }
.toc a { font-weight: 500; }

.form-group { margin-bottom: 20px; }
.form-label { display: block; margin-bottom: 8px; font-weight: 600; }
.form-control, .form-select { width: 100%; padding: 12px; border: 1px solid var(--border-color); border-radius: var(--radius-md); font-size: 1rem; }
.form-control:focus, .form-select:focus { outline: none; border-color: var(--primary-color); box-shadow: 0 0 0 3px rgba(30, 115, 190, 0.2); }
.calculator-box { background: var(--white); padding: 30px; border-radius: var(--radius-lg); box-shadow: var(--shadow-md); max-width: 600px; margin: 0 auto 40px; }
.result-box { background: #e8f4fd; border: 1px solid #b6d4fe; padding: 20px; border-radius: var(--radius-md); margin-top: 20px; text-align: center; display: none; }
.result-title { font-weight: 600; color: var(--secondary-color); margin-bottom: 10px; }
.result-value { font-size: 1.5rem; font-weight: 700; color: var(--primary-color); }

.faq-item { background: var(--white); border: 1px solid var(--border-color); border-radius: var(--radius-md); margin-bottom: 15px; overflow: hidden; }
.faq-question { padding: 15px 20px; font-weight: 600; cursor: pointer; background: #f8fbff; border-bottom: 1px solid var(--border-color); display: flex; justify-content: space-between; align-items: center; }
.faq-question::after { content: '+'; font-size: 1.5rem; color: var(--primary-color); }
.faq-item.active .faq-question::after { content: '-'; }
.faq-answer { padding: 20px; display: none; }
.faq-item.active .faq-answer { display: block; background: var(--white); }

footer { background: var(--secondary-color); color: var(--white); padding: 50px 0 20px; margin-top: 50px; }
.footer-grid { display: grid; grid-template-columns: 1fr; gap: 30px; margin-bottom: 30px; }
@media(min-width: 768px) { .footer-grid { grid-template-columns: repeat(3, 1fr); } }
.footer-title { margin-bottom: 20px; font-size: 1.2rem; }
.footer-links { list-style: none; padding: 0; }
.footer-links li { margin-bottom: 10px; }
.footer-links a { color: #ccc; }
.footer-links a:hover { color: var(--white); }
.footer-bottom { text-align: center; padding-top: 20px; border-top: 1px solid rgba(255,255,255,0.1); font-size: 0.9rem;}