How to Style Changed or Commented Text
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To style the highlighted text from the document content, use the <oxy-comment-hl> element (or <oxy-delete-hl>, <oxy-insert-hl>, respectively, by local name or class name):
oxy-comment-hl,
.oxy-comment-hl {
  color:magenta;
}
Note: The class name selector is useful when using the DITA Map PDF - based on HTML5 & CSS transformation scenario.

If you want to change the small labels that define the affected start and end ranges (they are a number formatted like: "[n] .. [/n]" where n is the number of the change), you can use the following selectors:

oxy-range-start:before,
.oxy-range-start:before {
  display:inline;
  content:'[' attr(hr_id) ']';
  color:red;
}
oxy-range-end:before,
.oxy-range-end:before {
  display:inline;
  content:'[/' attr(hr_id) ']';
  color:red;
}

Use display:none if you want to hide them.

If you want to change the background color of the comment footnote (assuming you set the show.changes.and.comments.as.pdf.sticky.notes transformation to no), add the following snippet in your customization CSS:

oxy-comment,
.oxy-comment {
  background-color:inherit;
  border: 2pt solid yellow;
}

Similarly, you can style the other footnotes for <oxy-attributes>, <oxy-delete>, and <oxy-insert> elements.