:root {
  --bg: #f5f7fa;
  --text: #333;
  --header-bg: #fff;
  --header-border: #ddd;
  --button-bg: #0066ff;
  --button-text: #fff;
}

body.dark {
  --bg: #1e1e1e;
  --text: #f5f5f5;
  --header-bg: #2b2b2b;
  --header-border: #444;
  --button-bg: #2196f3;
  --button-text: #fff;
}

body {
  margin: 0;
  font-family: system-ui, sans-serif;
  background: var(--bg);
  color: var(--text);
  transition: background 0.3s, color 0.3s;
}

header {
  background: var(--header-bg);
  border-bottom: 1px solid var(--header-border);
  padding: 1rem 0;
}

.container {
  width: 90%;
  max-width: 960px;
  margin: auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.logo {
  font-size: 1.5rem;
  font-weight: bold;
  color: var(--button-bg);
}

nav {
  display: flex;
  gap: 1rem;
  align-items: center;
}

.login-btn {
  text-decoration: none;
  padding: 0.5rem 1rem;
  background: var(--button-bg);
  color: var(--button-text);
  border-radius: 4px;
  transition: background 0.3s ease;
}

.login-btn:hover {
  background: #004fcc;
}

#themeToggle {
  background: none;
  border: none;
  font-size: 1.3rem;
  cursor: pointer;
  color: var(--text);
}

main {
  padding: 4rem 0;
  text-align: center;
}

.welcome h2 {
  margin-bottom: 1rem;
  font-size: 2rem;
}