* {
    box-sizing: border-box;
}
html,
body {
    min-height: 100%;
    margin: 0;
    padding: 0;
    font-family: 'Inter', 'Arial', sans-serif;
    background: #181A1B;
    color: #F6F7F8;
}
body {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    position: relative;
}
nav,
section,
footer {
    width: 100%;
}
/* Nav */
.nav-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 1.2em 5vw 1.2em 5vw;
    background: #181A1B;
    position: sticky;
    top: 0;
    z-index: 10;
    box-shadow: 0 3px 12px 0 rgba(24, 26, 27, 0.1);
}
.logo {
    display: flex;
    align-items: center;
    gap: 0.68em;
    font-size: 1.62em;
    font-weight: 900;
    letter-spacing: 0.03em;
    text-transform: uppercase;
    color: #FF1E28;
    text-decoration: none;
}
.logo-img {
    height: 1.8em;
    width: auto;
}
.nav-links {
    display: flex;
    align-items: center;
    gap: 2.5em;
}
.nav-link {
    color: #F6F7F8;
    text-decoration: none;
    font-weight: bold;
    font-size: 1.1em;
    letter-spacing: 0.01em;
    padding: 0.2em 0.2em;
    border-bottom: 2.5px solid transparent;
    transition: color 0.16s, border 0.16s;
}
.nav-link:hover,
.nav-link:focus {
    color: #FF1E28;
    border-bottom: 2.5px solid #FF1E28;
    outline: none;
}
@media (max-width: 900px) {
    .nav-links {
        gap: 1em;
    }
    .logo {
        font-size: 1.15em;
    }
}
@media (max-width: 640px) {
    .nav-bar {
        flex-direction: column;
        align-items: flex-start;
    }
    .nav-links {
        flex-wrap: wrap;
        gap: 1em;
        padding-bottom: 0.4em;
    }
}
/* Hero */
.hero {
    background: #181A1B;
    text-align: center;
    padding: 3.5em 1em 2.5em 1em;
    border-bottom: 4px solid #FF1E28;
}
.hero h1 {
    color: #FF1E28;
    font-size: 2.8em;
    font-weight: 900;
    margin-bottom: 0.3em;
    letter-spacing: -0.025em;
}
.hero p {
    color: #C0C2C9;
    font-size: 1.1em;
    max-width: 600px;
    margin: 1.2em auto 0.7em auto;
    line-height: 1.6;
}
/* About Course */
section {
    padding: 3em 5vw 1.5em 5vw;
    max-width: 900px;
    margin: 0 auto;
}
h2 {
    color: #FF1E28;
    font-size: 2rem;
    font-weight: 900;
    margin-bottom: 0.4em;
}
.about-text {
    color: #C0C2C9;
    font-size: 1.07em;
    line-height: 1.6;
    margin-bottom: 2em;
}
.lecture-info {
    color: #C0C2C9;
    font-size: 1.04em;
    margin-bottom: 3em;
}
.lecture-info b {
    color: #FF1E28;
    font-weight: 700;
}
/* Back button */
.detail-btn {
    background: #FF1E28;
    color: #F6F7F8;
    border: none;
    border-radius: 36px;
    padding: 0.65em 1.7em;
    font-weight: 700;
    font-size: 1em;
    cursor: pointer;
    transition: background 0.16s;
    text-decoration: none;
    display: inline-block;
    margin-bottom: 3rem;
}
.detail-btn:hover {
    background: #FFD700;
    color: #181A1B;
}
/* Footer */
.footer {
    width: 100%;
    text-align: center;
    padding: 1.8em 0 1.2em 0;
    font-size: 0.99em;
    color: #C0C2C9;
    opacity: 0.66;
    margin-top: auto;
}