:root {
  --bg: #f5f7fb;
  --card: #ffffff;
  --text: #1a1f2b;
  --accent: #0b6bcb;
  --accent-2: #2d8f5b;
  --error: #b42318;
  --border: #d0d7e2;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: "Segoe UI", Tahoma, sans-serif;
  background: radial-gradient(circle at top left, #e6f0ff, var(--bg));
  color: var(--text);
}

.container {
  max-width: 920px;
  margin: 2rem auto;
  padding: 0 1rem 2rem;
}

h1 {
  margin-bottom: 0.5rem;
}

h2 {
  margin: 0 0 0.8rem;
}

.intro {
  margin-top: 0;
  margin-bottom: 1.2rem;
}

.hint {
  margin: 0.9rem 0 0.2rem;
  font-size: 0.93rem;
  opacity: 0.9;
}

.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 1rem;
  margin-bottom: 1rem;
}

.toolbar {
  display: flex;
  flex-wrap: wrap;
  align-items: end;
  gap: 0.7rem;
}

label {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
  font-weight: 600;
}

input,
select,
button {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.52rem 0.65rem;
  font: inherit;
}

button {
  cursor: pointer;
  background: var(--accent);
  color: #fff;
  border: none;
  font-weight: 600;
}

button:hover {
  filter: brightness(0.95);
}

.secondary {
  background: var(--accent-2);
}

.hidden {
  display: none;
}

.forces {
  margin-top: 0.9rem;
  display: grid;
  gap: 0.6rem;
}

.fg-grid {
  margin-top: 0.6rem;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 0.6rem;
}

.force-row {
  display: grid;
  grid-template-columns: minmax(90px, 1fr) minmax(130px, 1.2fr) minmax(130px, 1.2fr) minmax(95px, 0.7fr) auto;
  gap: 0.5rem;
  align-items: end;
  background: #f7faff;
  border: 1px solid #dfe8f4;
  border-radius: 10px;
  padding: 0.55rem;
}

.unknown-cell input[type="checkbox"] {
  width: 1.1rem;
  height: 1.1rem;
}

.remove-btn {
  background: #c73636;
  padding: 0.5rem 0.7rem;
}

.error {
  color: var(--error);
  margin-top: 0.8rem;
  min-height: 1.2rem;
}

.results {
  margin-top: 0.8rem;
  padding: 0.8rem;
  background: #f1f7ff;
  border-radius: 8px;
}

.results p {
  margin: 0.35rem 0;
}

.steps .math-steps {
  margin: 0.4rem 0 0;
  padding: 0.7rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: #ffffff;
  font-size: 0.95rem;
  line-height: 1.5;
}

.steps .math-steps p {
  margin: 0.22rem 0;
}

.steps .math-steps .spacer {
  height: 0.35rem;
}

canvas {
  width: 100%;
  height: auto;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: linear-gradient(180deg, #fdfefe 0%, #f1f6fd 100%);
}

@media (max-width: 720px) {
  .force-row {
    grid-template-columns: 1fr 1fr;
  }

  .remove-btn {
    grid-column: span 2;
  }
}