style.css

:root {
  --bg: #f4f6f8;
  --surface: #ffffff;
  --surface-2: #eceff2;
  --border: #d7dce2;
  --text: #1b2430;
  --text-dim: #5a6779;
  --text-faint: #8b96a5;
  --accent: #1f6f78;
  --accent-strong: #14545c;
  --accent-contrast: #ffffff;
  --add-bg: #e2f5e9;
  --add-text: #146c2e;
  --add-border: #a9dfba;
  --del-bg: #fbe9ea;
  --del-text: #9a2530;
  --del-border: #f0b8bd;
  --focus-ring: #1f6f78;
  --shadow: 0 1px 2px rgba(20, 30, 45, 0.06), 0 8px 24px rgba(20, 30, 45, 0.05);
  --radius: 10px;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #14181e;
    --surface: #1b2027;
    --surface-2: #232a33;
    --border: #313b47;
    --text: #e7ecf2;
    --text-dim: #a3adba;
    --text-faint: #6d7889;
    --accent: #4fb3bd;
    --accent-strong: #6fc7d0;
    --accent-contrast: #08191b;
    --add-bg: #133023;
    --add-text: #6fdb9a;
    --add-border: #1f5c3d;
    --del-bg: #351c1f;
    --del-text: #ee9aa1;
    --del-border: #6b2c33;
    --focus-ring: #4fb3bd;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 8px 24px rgba(0, 0, 0, 0.35);
    color-scheme: dark;
  }
}

:root[data-theme="dark"] {
  --bg: #14181e;
  --surface: #1b2027;
  --surface-2: #232a33;
  --border: #313b47;
  --text: #e7ecf2;
  --text-dim: #a3adba;
  --text-faint: #6d7889;
  --accent: #4fb3bd;
  --accent-strong: #6fc7d0;
  --accent-contrast: #08191b;
  --add-bg: #133023;
  --add-text: #6fdb9a;
  --add-border: #1f5c3d;
  --del-bg: #351c1f;
  --del-text: #ee9aa1;
  --del-border: #6b2c33;
  --focus-ring: #4fb3bd;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 8px 24px rgba(0, 0, 0, 0.35);
  color-scheme: dark;
}

* {
  box-sizing: border-box;
}

html, body {
  background: var(--bg);
}

body {
  margin: 0;
  padding-inline: 20px;
  padding-block: 24px;
  font-family: "IBM Plex Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  color: var(--text);
  background: var(--bg);
}

.shell {
  max-width: 1180px;
  margin-inline: auto;
  display: flex;
  flex-direction: column;
  gap: 18px;
}

/* ---------- header ---------- */

.topbar {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}

.brand {
  display: flex;
  align-items: flex-start;
  gap: 12px;
}

.brand-mark {
  font-size: 28px;
  line-height: 1;
  color: var(--accent);
  padding-top: 2px;
}

.brand h1 {
  margin: 0;
  font-size: 22px;
  font-weight: 700;
  letter-spacing: -0.01em;
}

.tagline {
  margin: 4px 0 0;
  color: var(--text-dim);
  font-size: 13.5px;
  max-width: 46ch;
}

.repo-link {
  color: var(--text-dim);
  text-decoration: none;
  font-size: 13.5px;
  padding: 7px 12px;
  border: 1px solid var(--border);
  border-radius: 8px;
  white-space: nowrap;
}

.repo-link:hover {
  color: var(--text);
  border-color: var(--text-faint);
}

/* ---------- input panes ---------- */

.input-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 14px;
}

@media (max-width: 960px) {
  .input-grid {
    grid-template-columns: 1fr;
  }
}

.pane {
  display: flex;
  flex-direction: column;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  overflow: hidden;
}

.pane-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  padding: 10px 12px;
  border-bottom: 1px solid var(--border);
  background: var(--surface-2);
}

.pane-head label {
  font-weight: 600;
  font-size: 13px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.pane-actions {
  display: flex;
  align-items: center;
  gap: 8px;
  min-width: 0;
}

.file-name {
  font-size: 12px;
  color: var(--text-faint);
  max-width: 16ch;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.pane-text {
  width: 100%;
  min-height: 220px;
  resize: vertical;
  border: none;
  background: transparent;
  color: var(--text);
  font-family: "IBM Plex Mono", ui-monospace, Menlo, Consolas, monospace;
  font-size: 13px;
  line-height: 1.6;
  padding: 12px 14px;
  outline: none;
}

.pane-text::placeholder {
  color: var(--text-faint);
}

/* ---------- controls ---------- */

.controls {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
  padding: 10px 12px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

.control-group {
  display: inline-flex;
  border: 1px solid var(--border);
  border-radius: 8px;
  overflow: hidden;
}

.seg {
  appearance: none;
  border: none;
  background: transparent;
  color: var(--text-dim);
  font: inherit;
  font-size: 13px;
  padding: 7px 12px;
  cursor: pointer;
  border-right: 1px solid var(--border);
}

.seg:last-child {
  border-right: none;
}

.seg[aria-pressed="true"] {
  background: var(--accent);
  color: var(--accent-contrast);
  font-weight: 600;
}

.toggle {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 13px;
  color: var(--text-dim);
  white-space: nowrap;
  cursor: pointer;
}

.toggle input {
  accent-color: var(--accent);
}

.spacer {
  flex: 1 1 auto;
}

.btn-primary, .btn-ghost {
  font: inherit;
  font-size: 13.5px;
  font-weight: 600;
  border-radius: 8px;
  cursor: pointer;
  padding: 8px 16px;
}

.btn-primary {
  border: 1px solid var(--accent-strong);
  background: var(--accent);
  color: var(--accent-contrast);
}

.btn-primary:hover {
  background: var(--accent-strong);
}

.btn-ghost {
  border: 1px solid var(--border);
  background: transparent;
  color: var(--text-dim);
}

.btn-ghost:hover {
  color: var(--text);
  border-color: var(--text-faint);
}

button:focus-visible, input:focus-visible, textarea:focus-visible {
  outline: 2px solid var(--focus-ring);
  outline-offset: 2px;
}

/* ---------- result ---------- */

.result {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  overflow: hidden;
}

.result-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
  background: var(--surface-2);
  flex-wrap: wrap;
}

.summary {
  font-size: 13px;
  color: var(--text-dim);
  display: flex;
  gap: 14px;
  flex-wrap: wrap;
}

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

.summary .add-count {
  color: var(--add-text);
}

.summary .del-count {
  color: var(--del-text);
}

.result-actions {
  display: flex;
  gap: 8px;
}

.diff-viewport {
  overflow-x: auto;
  font-family: "IBM Plex Mono", ui-monospace, Menlo, Consolas, monospace;
  font-size: 13px;
  line-height: 1.7;
}

/* split view */

.split-table {
  width: 100%;
  border-collapse: collapse;
}

.split-table td {
  vertical-align: top;
  padding: 1px 10px;
  white-space: pre-wrap;
  word-break: break-word;
  border-left: 3px solid transparent;
}

.split-table .ln {
  width: 3.5em;
  min-width: 3.5em;
  text-align: right;
  color: var(--text-faint);
  user-select: none;
  padding-right: 10px;
  border-left: none;
}

.split-table td.col-left {
  width: 50%;
}

.split-table td.col-right {
  width: 50%;
  border-left: 1px solid var(--border);
}

.row-add .col-right {
  background: var(--add-bg);
  border-left-color: var(--add-border);
}

.row-del .col-left {
  background: var(--del-bg);
  border-left-color: var(--del-border);
}

.row-add .col-left, .row-del .col-right {
  background: var(--surface-2);
}

.row-empty {
  color: var(--text-faint);
}

.collapse-row td {
  padding: 6px 14px;
  color: var(--text-faint);
  font-style: italic;
  cursor: pointer;
  background: var(--surface-2);
  border-left: none;
}

.collapse-row:hover td {
  color: var(--text-dim);
}

/* unified view */

.unified {
  padding: 10px 14px;
}

.unified-line {
  padding: 1px 10px;
  white-space: pre-wrap;
  word-break: break-word;
  border-left: 3px solid transparent;
  display: flex;
  gap: 10px;
}

.unified-line .ln {
  width: 3.5em;
  min-width: 3.5em;
  text-align: right;
  color: var(--text-faint);
  user-select: none;
  flex: none;
}

.unified-line .ln.blank {
  visibility: hidden;
}

.unified-line.add {
  background: var(--add-bg);
  border-left-color: var(--add-border);
}

.unified-line.del {
  background: var(--del-bg);
  border-left-color: var(--del-border);
}

.unified-block {
  padding: 10px 14px;
}

ins.word-add, span.char-add {
  background: var(--add-bg);
  color: var(--add-text);
  text-decoration: none;
  border-radius: 3px;
  box-shadow: 0 0 0 1px var(--add-border);
}

del.word-del, span.char-del {
  background: var(--del-bg);
  color: var(--del-text);
  text-decoration: line-through;
  border-radius: 3px;
  box-shadow: 0 0 0 1px var(--del-border);
}

/* Character-level highlight WITHIN an already-tinted changed-line row (split table cell, or
   unified +/- line): needs a stronger fill than .word-add/.char-del's own add-bg/del-bg, which
   would be invisible sitting on a same-colored row background. Reuses the existing border tokens
   as a stronger fill rather than adding new ones. */
.intraline-add {
  background: var(--add-border);
  border-radius: 3px;
}

.intraline-del {
  background: var(--del-border);
  text-decoration: line-through;
  border-radius: 3px;
}

.empty-state {
  padding: 40px 20px;
  text-align: center;
  color: var(--text-faint);
}

/* ---------- footer ---------- */

.foot {
  text-align: center;
  color: var(--text-faint);
  font-size: 12.5px;
  padding-block: 6px 2px;
}

.foot a {
  color: var(--text-dim);
}

/* ---------- A -> B / A -> C comparison sections ---------- */

.pane-head .optional {
  font-weight: 400;
  text-transform: none;
  letter-spacing: 0;
  color: var(--text-faint);
}

/* A -> B and A -> C sit next to each other; they stack on narrow screens */
.diff-viewport.cmp-multi {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
}

.cmp-section {
  min-width: 0;
  overflow-x: auto;
}

.cmp-section + .cmp-section {
  border-left: 1px solid var(--border);
}

@media (max-width: 960px) {
  .diff-viewport.cmp-multi {
    grid-template-columns: 1fr;
  }

  .cmp-section + .cmp-section {
    border-left: none;
    border-top: 4px solid var(--border);
  }
}

.cmp-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  flex-wrap: wrap;
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
  background: var(--surface-2);
}

.cmp-head h2 {
  margin: 0;
  font-size: 14px;
  font-weight: 600;
  color: var(--text);
}