:root {
  color-scheme: dark;
  --text: #ffffff;
  --accent: #0078d4;
  --bg: #24292e;
  --nav-bg: #1a1f24;
  --section-bg: #1e2328;
  --footer-bg: #181c21;
  --font: system-ui, sans-serif;
  --space: 1rem;
  --max-width: 800px;
  --radius: 8px;
  --border: 1px;
  --transition: 0.3s ease;
  --shadow: 0 4px 8px rgba(0, 0, 0, 0.3);
}

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

html {
  scroll-behavior: smooth;
}

body {
  background-color: var(--bg);
  color: var(--text);
  line-height: 1.6;
  font-family: var(--font);
  padding: 0 var(--space);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

a {
  color: var(--text);
  text-decoration: none;
}

.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  border: 0;
  clip-path: inset(100%);
  white-space: nowrap;
}

.profile-image {
  aspect-ratio: 2 / 3;
  display: block;
  margin: 0 auto 1.5rem;
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  max-width: 100%;
  height: auto;
}

.course-badge {
  aspect-ratio: 1 / 1;
  max-width: 100%;
  height: auto;
  margin: 1rem auto;
  display: block;
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

nav {
  background-color: var(--nav-bg);
  border-bottom: var(--border) solid rgba(255, 255, 255, 0.1);
  padding: var(--space) 0;
}

nav ul {
  list-style: none;
  display: flex;
  justify-content: center;
  gap: 2rem;
  flex-wrap: wrap;
}

nav a {
  font-weight: bold;
  position: relative;
  padding: 0.5rem 1rem;
  transition: color var(--transition), background-color var(--transition);
}

nav a::after,
nav a[aria-current="page"]::before {
  content: "";
  position: absolute;
  left: 0;
  bottom: -4px;
  height: 2px;
  background-color: var(--accent);
  transition: width var(--transition);
}

nav a::after {
  width: 0;
}

nav a[aria-current="page"]::before {
  width: 100%;
}

nav a:hover,
nav a:focus {
  color: var(--accent);
}

nav a:hover {
  background-color: rgba(255, 255, 255, 0.05);
}

nav a:focus {
  outline: none;
}

nav a:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 4px;
  color: var(--accent);
}

main a:focus-visible,
footer nav a:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

header {
  text-align: center;
  padding: 2rem 0;
}

h1 {
  font-size: 2.5rem;
  margin-bottom: var(--space);
}

h2 {
  font-size: 1.5rem;
  border-bottom: var(--border) solid rgba(255, 255, 255, 0.3);
  padding-bottom: 0.5rem;
  margin-bottom: var(--space);
}

h3 {
  font-size: 1.2rem;
  margin-bottom: 0.5rem;
}

main {
  flex: 1;
  padding: var(--space);
  max-width: var(--max-width);
  margin: 0 auto;
  width: 100%;
}

main section {
  padding: var(--space);
  margin: var(--space) 0;
  background-color: var(--section-bg);
  border-radius: var(--radius);
}

.error-section {
  text-align: center;
  padding: 4rem var(--space);
  margin: auto;
  max-width: var(--max-width);
  background-color: var(--section-bg);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

.error-section h1 {
  font-size: 6rem;
  margin-bottom: var(--space);
  color: var(--accent);
  font-weight: bold;
}

.error-section h2 {
  font-size: 1.8rem;
  margin: 0;
  border: none;
  padding-bottom: 0;
}

.course-index {
  background-color: var(--nav-bg);
  padding: 1rem;
  border-radius: var(--radius);
  margin-bottom: 1rem;
  text-align: center;
}

.course-item {
  background-color: rgba(255, 255, 255, 0.05);
  border-radius: var(--radius);
  padding: 1rem;
  margin-bottom: 1.5rem;
  width: calc(100% - 2rem);
  margin-left: auto;
  margin-right: auto;
}

.skills-category {
  margin-bottom: 2rem;
  padding: 1rem;
  background-color: rgba(255, 255, 255, 0.05);
  border-radius: var(--radius);
}

.skills-category h3 {
  font-size: 1.1rem;
  margin-bottom: 0.75rem;
}

main a:hover,
main a:focus {
  color: var(--accent);
  text-decoration: underline;
}

ul {
  padding-left: 1.5rem;
  margin: 0.5rem 0;
}

ul li {
  margin-bottom: 0.5rem;
  line-height: 1.4;
}

p {
  margin: 1rem 0;
}

footer {
  background-color: var(--footer-bg);
  padding: var(--space) 0;
  text-align: center;
  font-size: 1rem;
  border-top: var(--border) solid rgba(255, 255, 255, 0.1);
}

footer p {
  margin-bottom: 0.5rem;
}

footer nav {
  display: inline-block;
  background-color: transparent;
  border: none;
  padding: 0;
}

footer nav a {
  color: var(--text);
  margin: 0 0.5rem;
  font-weight: normal;
  transition: none;
}

footer nav a:hover,
footer nav a:focus {
  color: var(--accent);
  text-decoration: underline;
}

footer nav a::after,
footer nav a[aria-current="page"]::before {
  display: none;
}

@media (max-width: 48em) {
  nav ul {
    gap: 0.75rem;
  }
  nav a {
    padding: 0.4rem 0.75rem;
    font-size: 0.9rem;
  }
  h1 {
    font-size: 2.2rem;
  }
  h2 {
    font-size: 1.2rem;
  }
}

@media (max-width: 30em) {
  nav ul {
    gap: 0.5rem;
  }
  nav a {
    font-size: 1em;
    padding: 0.5rem;
    width: auto;
    text-align: center;
  }
  h2 {
  font-size: 1.2rem;
  }
  .course-item {
    width: 100%;
  }
}