:root {
  --primary: #2563eb;
  --secondary: #f3f4f6;
  --dark: #1f2937;
  --radius: 0.75rem;
  --max: 960px;
}
*,
*::before,
*::after {
  box-sizing: border-box;
}
body {
  margin: 0;
  font-family: system-ui, sans-serif;
  line-height: 1.55;
  color: var(--dark);
  background: #fff;
}
header,
main,
footer,
section {
  max-width: var(--max);
  margin: auto;
  padding: 1.5rem 1rem;
}
#hero {
  background: var(--secondary);
  text-align: center;
  padding: 3rem 1rem;
}
h1,
h2,
h3 {
  margin: 0.5rem 0 1rem;
  font-weight: 700;
}
p {
  margin-bottom: 1rem;
}
.btn {
  display: inline-block;
  margin: 0.5rem 0.25rem;
  padding: 0.75rem 1.25rem;
  border-radius: var(--radius);
  background: var(--primary);
  color: #fff;
  text-decoration: none;
  font-weight: 600;
}

/* form tweaks */
form select,
form input[type="number"] {
  width: auto;
  max-width: 300px;
  display: block;
  margin: 0.5rem auto 1rem;
  padding: 0.5rem;
  border: 1px solid #d1d5db;
  border-radius: 0.5rem;
}

#selectorForm,
#roiFormCalc {
  text-align: center;
}

#channelGroup {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  justify-content: center;
  margin-bottom: 1rem;
}
#channelGroup label {
  font-weight: 400;
}

.card {
  border: 1px solid #e5e7eb;
  border-radius: var(--radius);
  padding: 1rem;
  margin: 1rem 0;
}

#roiResult {
  margin-top: 1rem;
  font-weight: 600;
}

#embed {
  text-align: center;
}
#embed code {
  display: inline-block;
  padding: 0.5rem;
  background: #f3f4f6;
  border-radius: 0.5rem;
  max-width: 100%;
  overflow-x: auto;
  font-family: monospace;
}

footer {
  background: #f9fafb;
  text-align: center;
  font-size: 0.875rem;
  color: #6b7280;
  border-top: 1px solid #e5e7eb;
}

/* utilities */
.hidden {
  display: none;
}

/* NEW */
.error {
  color: #b91c1c; /* red-600 */
  background: #fef2f2; /* red-50 */
  border: 1px solid #fecaca; /* red-200 */
  padding: 1rem;
  border-radius: 0.75rem;
  margin: 1rem 0;
}

.link-list {
  list-style: none;
  padding: 0;
}
.link-list li {
  margin: 0.5rem 0;
}