:root {
  --bg: #0f172a;
  --bg-panel: #1e293b;
  --bg-panel-2: #273449;
  --border: #334155;
  --text: #e2e8f0;
  --text-dim: #94a3b8;
  --accent: #38bdf8;
  --accent-2: #a78bfa;
  --ok: #4ade80;
  --bad: #f87171;
  --mono: "Consolas", "SFMono-Regular", Menlo, monospace;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, "Segoe UI", Roboto, sans-serif;
  line-height: 1.5;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

header.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 24px;
  background: var(--bg-panel);
  border-bottom: 1px solid var(--border);
}

header.topbar .brand { font-weight: 700; font-size: 1.1rem; }
header.topbar nav { display: flex; flex-wrap: wrap; justify-content: flex-end; gap: 4px 0; }
header.topbar nav a { margin-left: 18px; color: var(--text-dim); font-size: 0.92rem; }
header.topbar nav a:hover { color: var(--text); }

main {
  max-width: 980px;
  margin: 0 auto;
  padding: 28px 20px 60px;
}

h1 { font-size: 1.6rem; margin-bottom: 4px; }
h2 { font-size: 1.2rem; margin-top: 0; }
.subtitle { color: var(--text-dim); margin-top: 0; margin-bottom: 24px; }

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 16px;
  margin-top: 20px;
}

.card {
  display: block;
  background: var(--bg-panel);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 18px;
  transition: border-color 0.15s;
}
.card:hover { border-color: var(--accent); text-decoration: none; }
.card h3 { margin: 0 0 6px; color: var(--text); }
.card p { margin: 0; color: var(--text-dim); font-size: 0.9rem; }

.panel {
  background: var(--bg-panel);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px;
  margin-bottom: 20px;
}

.form-row { margin-bottom: 14px; }
.form-row label { display: block; margin-bottom: 5px; font-size: 0.88rem; color: var(--text-dim); }
.form-row input[type=text], .form-row input[type=number], .form-row textarea, .form-row select {
  width: 100%;
  padding: 9px 10px;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--text);
  font-family: var(--mono);
  font-size: 0.95rem;
}
.form-row textarea { min-height: 70px; resize: vertical; }
.form-inline { display: flex; gap: 12px; flex-wrap: wrap; }
.form-inline .form-row { flex: 1; min-width: 140px; }

.btn {
  display: inline-block;
  padding: 9px 16px;
  background: var(--accent);
  color: #0b1120;
  border: none;
  border-radius: 6px;
  font-weight: 600;
  cursor: pointer;
  font-size: 0.9rem;
}
.btn:hover { filter: brightness(1.1); }
.btn.secondary { background: var(--bg-panel-2); color: var(--text); border: 1px solid var(--border); }
.btn.ghost { background: transparent; color: var(--accent); border: 1px solid var(--accent); }
.btn-row { display: flex; gap: 10px; flex-wrap: wrap; margin-top: 10px; }

.mode-toggle { display: flex; gap: 8px; margin-bottom: 16px; }
.mode-toggle button {
  padding: 7px 14px; border-radius: 999px; border: 1px solid var(--border);
  background: var(--bg-panel-2); color: var(--text-dim); cursor: pointer; font-size: 0.85rem;
}
.mode-toggle button.active { background: var(--accent); color: #0b1120; border-color: var(--accent); font-weight: 600; }

.result-box {
  background: var(--bg);
  border: 1px solid var(--accent);
  border-radius: 8px;
  padding: 14px 16px;
  margin-bottom: 20px;
  font-family: var(--mono);
  font-size: 1.05rem;
  word-break: break-all;
}
.result-box .label { display: block; font-family: inherit; font-size: 0.8rem; color: var(--text-dim); margin-bottom: 4px; }
.result-box.result-fail { border-color: var(--bad); }
.result-box.result-fail .label { color: #fca5a5; }

.step {
  border-left: 3px solid var(--accent);
  padding: 10px 0 10px 16px;
  margin-bottom: 16px;
}
.step.step-bad { border-left-color: var(--bad); }
.step .step-title { font-weight: 600; margin-bottom: 4px; }
.step .step-title .idx { color: var(--text-dim); font-weight: 400; margin-right: 6px; }
.step .step-explanation { color: var(--text-dim); font-size: 0.9rem; margin-bottom: 8px; white-space: pre-wrap; }
.step .step-formula {
  font-family: var(--mono); background: var(--bg); border: 1px solid var(--border);
  border-radius: 6px; padding: 6px 10px; display: inline-block; margin-bottom: 8px; font-size: 0.9rem;
}

table.step-table { width: 100%; table-layout: fixed; border-collapse: collapse; font-family: var(--mono); font-size: 0.85rem; margin-top: 6px; }
table.step-table th, table.step-table td { border: 1px solid var(--border); padding: 5px 8px; text-align: center; word-break: break-word; overflow-wrap: anywhere; }
table.step-table th { background: var(--bg-panel-2); color: var(--text-dim); font-weight: 600; }
table.step-table tbody tr:nth-child(even) { background: rgba(255,255,255,0.02); }

.alert { padding: 12px 16px; border-radius: 8px; margin-bottom: 16px; font-size: 0.9rem; }
.alert.error { background: rgba(248,113,113,0.12); border: 1px solid var(--bad); color: #fecaca; }
.alert.success { background: rgba(74,222,128,0.1); border: 1px solid var(--ok); color: #bbf7d0; }

.note-real-world {
  margin-top: 24px; padding: 14px 16px; border-radius: 8px;
  background: rgba(167,139,250,0.08); border: 1px solid var(--accent-2); font-size: 0.85rem; color: #ddd6fe;
}

.badge { display: inline-block; padding: 2px 8px; border-radius: 999px; font-size: 0.72rem; background: var(--bg-panel-2); color: var(--text-dim); margin-left: 8px; }

.history-table { width: 100%; border-collapse: collapse; font-size: 0.88rem; }
.history-table th, .history-table td { border-bottom: 1px solid var(--border); padding: 8px 10px; text-align: left; }
.history-table th { color: var(--text-dim); font-weight: 600; }
.history-table tr { cursor: pointer; }
.history-table tr:hover { background: var(--bg-panel-2); }

.filters { display: flex; gap: 10px; margin-bottom: 16px; flex-wrap: wrap; }
.filters select { padding: 7px 10px; background: var(--bg-panel); border: 1px solid var(--border); border-radius: 6px; color: var(--text); }

.htmx-indicator { display: none; color: var(--text-dim); font-size: 0.85rem; }
.htmx-request .htmx-indicator { display: inline; }
.htmx-request.htmx-indicator { display: inline; }

.example-btn { font-size: 0.8rem; color: var(--text-dim); background: none; border: 1px dashed var(--border); border-radius: 6px; padding: 5px 10px; cursor: pointer; }
.example-btn:hover { border-color: var(--accent); color: var(--accent); }

/* Python-Tutor-style code execution visualizer */
.tutor-toolbar {
  display: flex; align-items: center; gap: 8px; flex-wrap: wrap;
  background: var(--bg-panel); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px 14px; margin-bottom: 16px;
}
.tutor-counter { font-family: var(--mono); font-size: 0.85rem; color: var(--text-dim); white-space: nowrap; }
.tutor-speed { background: var(--bg); border: 1px solid var(--border); border-radius: 6px; color: var(--text); padding: 6px 8px; }
.tutor-panes { display: grid; grid-template-columns: 1.3fr 1fr; gap: 16px; }
@media (max-width: 800px) { .tutor-panes { grid-template-columns: 1fr; } }
.tutor-pane { background: var(--bg-panel); border: 1px solid var(--border); border-radius: 10px; padding: 14px; height: 560px; min-width: 0; display: flex; flex-direction: column; overflow: hidden; }
.tutor-pane-label { font-size: 0.8rem; color: var(--text-dim); margin-bottom: 10px; display: flex; align-items: center; gap: 8px; font-family: var(--mono); flex: none; }
.tutor-source { margin: 0; overflow-x: auto; overflow-y: auto; flex: 1; min-height: 0; font-family: var(--mono); font-size: 0.86rem; line-height: 1.6; }
.tutor-vars { overflow-y: auto; overflow-x: hidden; min-width: 0; }
.tutor-vars .step-table td { text-align: left; }
.tutor-vars .step-table td:first-child { width: 30%; }
.tutor-line { display: flex; gap: 12px; padding: 1px 6px; border-radius: 4px; white-space: pre; }
.tutor-line-active { background: rgba(56,189,248,0.16); outline: 1px solid var(--accent); }
.tutor-line-no { color: var(--text-dim); min-width: 2.2em; text-align: right; user-select: none; }
.tutor-vars .step-table { margin-top: 0; }
.tutor-return { font-family: var(--mono); background: rgba(74,222,128,0.1); border: 1px solid var(--ok); border-radius: 6px; padding: 8px 10px; margin-bottom: 10px; color: #bbf7d0; }

/* Bar chart (letter frequencies) */
.bar-chart {
  display: flex;
  align-items: flex-end;
  gap: 6px;
  height: 140px;
  padding: 10px 6px 0;
  margin: 10px 0 16px;
  border-bottom: 1px solid var(--border);
  overflow-x: auto;
}
.bar-col { display: flex; flex-direction: column; align-items: center; min-width: 22px; height: 100%; justify-content: flex-end; }
.bar-track { flex: 1; display: flex; align-items: flex-end; width: 16px; }
.bar-chart .bar { width: 100%; min-height: 2px; background: linear-gradient(180deg, var(--accent), var(--accent-2)); border-radius: 3px 3px 0 0; }
.bar-label { font-family: var(--mono); font-size: 0.72rem; color: var(--text-dim); margin-top: 4px; }
.bar-value { font-family: var(--mono); font-size: 0.68rem; color: var(--text); }

/* Feistel diagram (S-DES) */
.feistel-diagram { margin: 14px 0; }
.fd-row, .fd-halves, .fd-flow { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; margin-bottom: 10px; }
.fd-halves { justify-content: center; }
.fd-box {
  background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 8px 12px; text-align: center; font-family: var(--mono);
}
.fd-box-label { font-size: 0.72rem; color: var(--text-dim); }
.fd-box-sub { font-size: 0.68rem; color: var(--text-dim); }
.fd-box-value { font-size: 0.95rem; font-weight: 600; color: var(--text); }
.fd-io { border-color: var(--accent); }
.fd-half { border-color: var(--accent-2); min-width: 70px; }
.fd-dim { opacity: 0.55; }
.fd-f { border-color: var(--ok); border-style: dashed; }
.fd-final { border-width: 2px; }
.fd-arrow { color: var(--text-dim); font-size: 0.8rem; white-space: nowrap; }
.fd-swap { font-size: 0.9rem; color: var(--accent); text-align: center; width: 100%; justify-content: center; }
.fd-round { background: var(--bg-panel-2); border-radius: 8px; padding: 10px 12px; margin-bottom: 14px; }
.fd-round-label { font-size: 0.78rem; color: var(--text-dim); margin-bottom: 6px; text-transform: uppercase; letter-spacing: 0.04em; }
.fd-note { font-size: 0.78rem; color: var(--text-dim); margin-top: 4px; }

/* Math explainer */
details.math-explainer {
  margin-top: 16px; padding: 14px 16px; border-radius: 8px;
  background: rgba(56,189,248,0.06); border: 1px solid var(--accent);
}
details.math-explainer summary { cursor: pointer; font-weight: 600; color: var(--accent); }
details.math-explainer[open] summary { margin-bottom: 8px; }
details.math-explainer p { font-size: 0.88rem; color: var(--text); margin: 8px 0; }
details.math-explainer .formula-block { font-family: var(--mono); background: var(--bg); border: 1px solid var(--border); border-radius: 6px; padding: 8px 10px; margin: 8px 0; font-size: 0.88rem; }