:root {
    --primary-color: #2e7d32; /* A richer, more academic green */
    --secondary-color: #1976d2; /* A friendly, trustworthy blue */
    --background-color: #fcfcfc;
    --text-color: #37474f;
    --text-color-light: #607d8b;
    --card-bg: #ffffff;
    --border-color: #eceff1;
    --header-bg: rgba(255, 255, 255, 0.85);
    --font-sans: 'Noto Sans KR', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
}

body.dark-mode {
    --primary-color: #81c784;
    --secondary-color: #64b5f6;
    --background-color: #1c1c1c;
    --text-color: #cfd8dc;
    --text-color-light: #90a4ae;
    --card-bg: #282828;
    --border-color: #373737;
    --header-bg: rgba(30, 30, 30, 0.85);
}

/* --- General & Typography --- */
body {
    font-family: var(--font-sans);
    background-color: var(--background-color);
    color: var(--text-color);
    margin: 0;
    line-height: 1.7;
    transition: background-color 0.3s, color 0.3s;
}
h1, h2, h3, h4, h5, h6 {
    font-weight: 700;
    color: var(--primary-color);
    line-height: 1.3;
}
h1 { font-size: 3rem; }
h2 { font-size: 2.25rem; }
h3 { font-size: 1.5rem; }
p { color: var(--text-color-light); }
a {
    color: var(--secondary-color);
    text-decoration: none;
    transition: color 0.2s;
}
a:hover { color: var(--primary-color); }
.container {
    width: 90%;
    max-width: 1100px;
    margin-left: auto;
    margin-right: auto;
}
.section-title { text-align: center; margin-bottom: 3rem; }

/* --- Header & Footer --- */
header {
    background-color: var(--header-bg);
    backdrop-filter: blur(10px);
    box-shadow: 0 1px 3px rgba(0,0,0,0.05);
    position: sticky;
    top: 0;
    width: 100%;
    z-index: 1000;
    padding: 1rem 0;
}
header .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
}
.site-title {
    font-size: 1.5rem;
    font-weight: 800;
    color: var(--primary-color);
}
header nav a {
    color: var(--text-color);
    margin: 0 1.5rem;
    font-size: 1.1rem;
    font-weight: 500;
}
#theme-toggle {
    background: var(--text-color);
    color: var(--background-color);
    border: none;
    padding: 0.6rem 1.2rem;
    border-radius: 30px;
    cursor: pointer;
    font-weight: 600;
    font-size: 0.9rem;
}
footer {
    border-top: 1px solid var(--border-color);
    text-align: center;
    padding: 3rem 2rem;
    margin-top: 4rem;
    color: var(--text-color-light);
}
footer .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
}
footer nav a { margin: 0 1rem; color: var(--text-color-light); }

/* --- Reusable Components --- */
.card {
    background-color: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 2rem;
    transition: transform 0.3s, box-shadow 0.3s;
    display: block; /* For <a> tags */
}
.card:hover {
    transform: translateY(-8px);
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.08);
}
.grid-container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: 1.5rem;
}
.disabled {
    opacity: 0.6;
    pointer-events: none;
}

/* --- Homepage Sections --- */
.hero {
    text-align: center;
    padding: 5rem 2rem;
}
.hero p {
    font-size: 1.25rem;
    max-width: 650px;
    margin: 1rem auto 0;
}
.levels { padding: 2rem 0; }
.level-card .level-tag {
    display: inline-block;
    padding: 0.3rem 0.8rem;
    background-color: var(--primary-color);
    color: #fff;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: 700;
    margin-bottom: 1rem;
}
.level-card h3 { margin-bottom: 0.5rem; }
.today-expression { padding: 4rem 0; background-color: var(--card-bg); }

/* --- Learning Page Specifics --- */
.page-header {
    text-align: center;
    padding: 4rem 2rem;
    background-color: var(--card-bg);
    border-bottom: 1px solid var(--border-color);
}
.page-header p { font-size: 1.1rem; max-width: 700px; margin: 1rem auto 0; }
.content-section {
    padding: 3rem 0;
    border-bottom: 1px solid var(--border-color);
}
.content-section:last-child { border-bottom: none; }

.expression-card {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 1.5rem;
    margin-bottom: 1rem;
}
.expression-card .korean {
    font-size: 1.5rem;
    font-weight: 600;
}
.expression-card .romanization { font-style: italic; color: var(--secondary-color); }
.tts-button {
    background: var(--primary-color);
    color: #fff;
    border: none;
    border-radius: 50%;
    width: 44px;
    height: 44px;
    font-size: 1.5rem;
    cursor: pointer;
    line-height: 44px;
    text-align: center;
}
.hidden { display: none; }
.example-sentence {
    background-color: var(--background-color);
    border-left: 4px solid var(--secondary-color);
    padding: 1rem;
    margin-top: 1rem;
    font-style: italic;
}
.toggle-example { cursor: pointer; font-weight: 600; }