This style works with gmail-generated quoted text blocks, and works best when people intersperse comments in replies, so the new text stands out. The quoted text at deeper levels is given a varying off-gray, slightly lighter color, and the deepest levels are just gray. The size of the fonts is also decreased slightly.

There is also some correction of grayed text at the top level that should not have been marked with quotes.

Only quotes that are recognized by gmail and marked as such will be affected - they show up by default with a left border edge. Quotes that are still prefixed by the commonly used angle bracket (>) are not distinguished from other text in the markup so I can't apply a style to them, unfortunately. (Some JS would be required to convert such text first.)