* {
  box-sizing: border-box;
}

:root {
  --bg: #070b14;
  --panel: rgba(15, 23, 42, 0.92);
  --soft: rgba(255, 255, 255, 0.06);
  --border: rgba(255, 255, 255, 0.13);
  --text: #eef4ff;
  --muted: #94a3b8;
  --green: #32d583;
  --blue: #5b8cff;
  --purple: #9b7cff;
}

body {
  margin: 0;
  min-height: 100vh;
  font-family: Inter, Arial, sans-serif;
  color: var(--text);
  background:
    radial-gradient(circle at 20% 20%, rgba(91, 140, 255, 0.25), transparent 30%),
    radial-gradient(circle at 82% 12%, rgba(50, 213, 131, 0.17), transparent 32%),
    radial-gradient(circle at 50% 95%, rgba(155, 124, 255, 0.14), transparent 34%),
    var(--bg);
}

.page {
  width: min(980px, 100%);
  margin: auto;
  padding: 28px;
}

.top-actions {
  display: flex;
  justify-content: space-between;
  gap: 14px;
  margin-bottom: 24px;
}

.back,
.learn-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 46px;
  padding: 12px 18px;
  border-radius: 999px;
  color: var(--text);
  text-decoration: none;
  background: rgba(255, 255, 255, 0.08);
  border: 1px solid var(--border);
  font-weight: 900;
}

.learn-btn {
  background: linear-gradient(135deg, var(--blue), var(--purple));
  border: 0;
}

.panel {
  padding: 34px;
  border-radius: 34px;
  background: linear-gradient(145deg, rgba(15, 23, 42, 0.96), rgba(17, 24, 39, 0.84));
  border: 1px solid var(--border);
  box-shadow: 0 30px 90px rgba(0, 0, 0, 0.45);
}

.eyebrow {
  margin: 0 0 10px;
  color: var(--green);
  font-size: 13px;
  font-weight: 900;
  letter-spacing: 0.14em;
  text-transform: uppercase;
}

h1 {
  margin: 0;
  font-size: clamp(38px, 7vw, 66px);
  line-height: 0.95;
  letter-spacing: -0.06em;
}

.muted {
  color: var(--muted);
  line-height: 1.6;
}

.mode-selector {
  margin-top: 26px;
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  gap: 10px;
  padding: 10px;
  border-radius: 24px;
  background: rgba(255, 255, 255, 0.045);
  border: 1px solid var(--border);
}

.mode,
button {
  width: 100%;
  min-height: 52px;
  padding: 12px 14px;
  border-radius: 16px;
  border: 0;
  font-size: 15px;
  font-weight: 900;
  cursor: pointer;
  transition: 0.2s ease;
}

.mode {
  background: transparent;
  color: var(--muted);
  border: 1px solid transparent;
}

.mode:hover {
  color: var(--text);
  background: rgba(255, 255, 255, 0.07);
}

.mode.active {
  color: white;
  background: linear-gradient(135deg, var(--blue), var(--purple));
  box-shadow: 0 16px 36px rgba(91, 140, 255, 0.28);
}

.task-card,
.result-box {
  margin-top: 24px;
  padding: 28px;
  border-radius: 28px;
  background: var(--soft);
  border: 1px solid var(--border);
}

.pill {
  display: inline-block;
  margin: 0 0 12px;
  padding: 8px 13px;
  border-radius: 999px;
  color: #b9ccff;
  background: rgba(91, 140, 255, 0.14);
  font-weight: 900;
}

.task-type {
  margin: 0 0 8px;
  color: var(--green);
  font-weight: 900;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  font-size: 12px;
}

.task-card h2 {
  margin: 0 0 18px;
  font-size: 28px;
  letter-spacing: -0.03em;
  line-height: 1.35;
}

input {
  width: 100%;
  height: 58px;
  margin-bottom: 14px;
  padding: 0 18px;
  border-radius: 18px;
  border: 1px solid var(--border);
  background: rgba(7, 11, 20, 0.72);
  color: var(--text);
  font-size: 17px;
  outline: none;
}

input:focus {
  border-color: var(--blue);
  box-shadow: 0 0 0 4px rgba(91, 140, 255, 0.16);
}

button {
  margin-top: 14px;
  padding: 15px;
  border-radius: 18px;
  background: linear-gradient(135deg, var(--green), #12b76a);
  color: #03130b;
}

.secondary {
  background: rgba(255, 255, 255, 0.08);
  color: var(--text);
  border: 1px solid var(--border);
}

button:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

#feedback {
  min-height: 28px;
  margin: 20px 0 6px;
  font-size: 20px;
  font-weight: 900;
}

.formula-box {
  margin-top: 16px;
  padding: 18px;
  border-radius: 20px;
  background: rgba(91, 140, 255, 0.13);
  border: 1px solid rgba(91, 140, 255, 0.35);
}

#formulaText {
  margin: 0;
  font-size: 17px;
  font-weight: 900;
  line-height: 1.5;
}

.hidden {
  display: none;
}

.result-box h2 {
  margin: 0 0 16px;
}

.result-box p {
  color: var(--muted);
}

.result-box strong {
  color: var(--text);
}

ul {
  padding: 0;
  margin: 16px 0 0;
  list-style: none;
}

li {
  margin: 10px 0;
  padding: 16px;
  border-radius: 16px;
  background: rgba(7, 11, 20, 0.45);
  border: 1px solid var(--border);
  line-height: 1.6;
}

.result-title {
  display: block;
  margin-bottom: 6px;
  font-weight: 900;
}

.result-meta {
  color: var(--muted);
  font-size: 14px;
}

.result-ok {
  color: #32d583;
}

.result-bad {
  color: #ff7d95;
}

#correctWrongBtn {
  margin-top: 20px;
}

@media (max-width: 880px) {
  .mode-selector {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (max-width: 700px) {
  .page {
    padding: 20px 14px;
  }

  .top-actions {
    flex-direction: column;
  }

  .back,
  .learn-btn {
    width: 100%;
  }

  .panel {
    padding: 22px;
    border-radius: 26px;
  }

  .task-card,
  .result-box {
    padding: 20px;
  }

  .mode-selector {
    grid-template-columns: 1fr;
  }
}