Seems to highlight new comments, but I can't tell how new. I think newer than (including) 10 minutes.

New comments since you last visited I believe?

Edit: It's comments newer than 10 minutes on default lemmy-ui

Lighter shades should be comments that have been added since you opened the thread iirc

Would be cool if there was a way that new comments were highlighted regardless of time. Much like how Apollo does it on Reddit.

I think the lighter comments are new comments since the last time you visited that post?

It means those are new from the last time you viewed the thread. So if you see a thread that says (3 new) the new ones will have that greyed background