message: Use more visible color for `<del>` in markdown
  Fixes
diff: /* Lighten deletions in Markdown */
  .markdown-body del {
  - color: var(--color-fg-subtle, var(--color-text-tertiary, #6a737d));
  + color: var(--color-fg-muted);
  }
  /* Make <details> visibly clickable */
  