trpl-zh-cn/theme/semantic-notes.css

/*
  This is copied directly from the styles for blockquotes, because notes were
  historically rendered *as* blockquotes. This keeps the presentation of them
  identical while updating the presentation.
*/
.note {
  margin: 20px 0;
  padding: 0 20px;
  color: var(--fg);
  background-color: var(--quote-bg);
  border-block-start: 0.1em solid var(--quote-border);
  border-block-end: 0.1em solid var(--quote-border);
}