/* Basic Styles */
:root {
  --primary-color: #2c3e50;
  --secondary-color: #3498db;
  --bg-color: #f4f7f6;
  --card-bg: #ffffff;
  --text-color: #333333;
}

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

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

.container {
  max-width: 900px;
  margin: 0 auto;
  padding: 0 20px;
}

/* Header */
header {
  background-color: var(--primary-color);
  color: #fff;
  padding: 60px 0;
  text-align: center;
}

header h1 {
  margin-bottom: 10px;
  font-size: 2.5rem;
}

/* Navigation */
nav {
  background: #fff;
  border-bottom: 1px solid #ddd;
  position: sticky;
  top: 0;
  z-index: 100;
}

nav ul {
  display: flex;
  justify-content: center;
  list-style: none;
  padding: 15px 0;
}

nav ul li {
  margin: 0 20px;
}

nav ul li a {
  text-decoration: none;
  color: var(--primary-color);
  font-weight: bold;
  transition: color 0.3s;
}

nav ul li a:hover {
  color: var(--secondary-color);
}

/* Sections */
section {
  padding: 60px 0;
}

section h2 {
  margin-bottom: 30px;
  border-left: 5px solid var(--secondary-color);
  padding-left: 15px;
  font-size: 1.8rem;
}

.card {
  background: var(--card-bg);
  padding: 30px;
  border-radius: 10px;
  box-shadow: 0 4px 6px rgba(0,0,0,0.1);
  margin-bottom: 20px;
}

.card h3 {
  margin-bottom: 10px;
  color: var(--primary-color);
}

.subtitle {
  color: #777;
  font-size: 0.9rem;
  margin-bottom: 15px;
}

ul {
  padding-left: 20px;
}

/* Project Grid */
.project-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
  gap: 20px;
}

.project-card {
  background: var(--card-bg);
  padding: 20px;
  border-radius: 10px;
  box-shadow: 0 4px 6px rgba(0,0,0,0.1);
  transition: transform 0.3s, box-shadow 0.3s;
  display: flex;
  flex-direction: column;
}

.project-card:hover {
  transform: translateY(-5px);
  box-shadow: 0 10px 20px rgba(0,0,0,0.15);
}

.project-card h3 {
  margin-bottom: 10px;
}

.btn {
  display: inline-block;
  background-color: var(--secondary-color);
  color: #fff;
  padding: 10px 20px;
  text-decoration: none;
  border-radius: 5px;
  margin-top: auto;
  text-align: center;
  transition: background 0.3s;
}

.btn:hover {
  background-color: #2980b9;
}

/* Form Styles */
.contact-form {
  display: flex;
  flex-direction: column;
}

.form-group {
  margin-bottom: 20px;
}

.form-group label {
  display: block;
  margin-bottom: 8px;
  font-weight: bold;
}

.form-group input,
.form-group textarea {
  width: 100%;
  padding: 12px;
  border: 1px solid #ddd;
  border-radius: 5px;
  font-size: 1rem;
  font-family: inherit;
}

.form-group input:focus,
.form-group textarea:focus {
  outline: none;
  border-color: var(--secondary-color);
  box-shadow: 0 0 5px rgba(52, 152, 219, 0.3);
}

.contact-form .btn {
  width: 100%;
  font-size: 1.1rem;
  cursor: pointer;
  border: none;
}

/* Footer */
footer {
  text-align: center;
  padding: 40px 0;
  background-color: var(--primary-color);
  color: #fff;
  margin-top: 40px;
}

/* Back Button Container */
.back-btn-container {
  text-align: center;
  margin-top: 30px;
}

/* Detail Page Specific */
.period {
  font-weight: bold;
  opacity: 0.9;
}

@media (max-width: 600px) {
  header h1 {
    font-size: 1.8rem;
  }
  nav ul li {
    margin: 0 10px;
  }
}