:root {
  --blue: #2563EB;
  --bg: #F8FAFC;
  --card: #FFFFFF;
  --text: #1E293B;
  --muted: #64748B;
  --border: #E2E8F0;
  --green: #16A34A;
  --red: #DC2626;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, "Segoe UI", Roboto, sans-serif;
  background: var(--bg);
  color: var(--text);
}

nav {
  background: var(--card);
  border-bottom: 1px solid var(--border);
  padding: 16px 24px;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

nav a { color: var(--blue); text-decoration: none; font-weight: 600; }

.link-button {
  background: none;
  border: none;
  color: var(--blue);
  font-weight: 600;
  cursor: pointer;
  padding: 0;
  font-size: inherit;
  width: auto;
  margin: 0;
}

.container {
  max-width: 900px;
  margin: 32px auto;
  padding: 0 16px;
}

.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 16px;
  padding: 24px;
  margin-bottom: 16px;
}

h1, h2 { color: var(--text); }

.btn {
  display: inline-block;
  background: var(--blue);
  color: #fff;
  border: none;
  border-radius: 10px;
  padding: 10px 20px;
  font-size: 15px;
  cursor: pointer;
  text-decoration: none;
}

.btn:hover { opacity: 0.9; }
.btn-secondary { background: var(--muted); }

table { width: 100%; border-collapse: collapse; }
th, td { text-align: left; padding: 10px 8px; border-bottom: 1px solid var(--border); }

.badge { padding: 4px 10px; border-radius: 999px; font-size: 13px; }
.badge-graded { background: #DCFCE7; color: var(--green); }
.badge-submitted { background: #FEF9C3; color: #A16207; }
.badge-none { background: #F1F5F9; color: var(--muted); }

.messages { list-style: none; padding: 0; }
.messages li { background: #DCFCE7; color: var(--green); padding: 10px 16px; border-radius: 10px; margin-bottom: 12px; }

input, textarea, select {
  width: 100%;
  padding: 10px;
  border: 1px solid var(--border);
  border-radius: 10px;
  margin-bottom: 12px;
  font-size: 15px;
}

label { font-weight: 600; font-size: 14px; }

.errorlist { color: var(--red); list-style: none; padding: 0; margin: -8px 0 12px; }