message: Slight improvements to HTML export CSS
  Use rounded backgrounds for context strings. Use system font where
  supported. This also affects macOS Quicklook previews.
diff: @@ -244,7 +244,7 @@ Based on Minimal CSS (minimalcss.com) under the MIT license.
  * { margin: 0; padding: 0; -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; }
  /* Layout */
  -body { background-color: #fff; color: #333; font: 14px/20px "HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif; }
  +body { background-color: #fff; color: #333; font: 14px/20px -apple-system, "HelveticaNeue", "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif; }
  header { width: 100%; margin: 0 auto; position: relative; padding: 20px 0; margin-bottom: 20px; }
  @@ -372,7 +372,8 @@ tr.comments div p:last-child { margin-bottom: 0; }
  color: rgb(70, 109, 137);
  font-size: smaller;
  background-color: rgb(217, 232, 242);
  - padding: 1px 2px;
  + border-radius: 2px;
  + padding: 2px 4px;
  margin-right: 4px;
  }
  