When I could use Turbo C on my own computer in the 1990s, syntax highlighting just came along for the ride. I learned vi from Watcom. I used vim. I wrote in perl, and PHP. I also learned Emacs. I wrote in Common Lisp. Syntax highlighting just stayed on.
And then I saw someone do something I had never seen before, and I wasn't sure how to do, and I noticed they weren't using syntax highlighting.
So I turned it off.
I would sometimes use ed to make changes to programs since I can see what I was working on before I needed to make a change, so I don't think it was doing anything for me, but at the same time I don't think I ever thought about syntax highlighting until I tried to work with dense code.
I do try consciously to improve myself, but nobody knows how they program and how to get better. The only strategy that has worked reliably: If I see someone do something I cannot do I try their methods until it makes sense in my own mind.
I've tried to write about this previously[2], but most programmers are hostile when seeing someone do something they can't do, rather than just trying it, they make up all sorts of reasons why they don't need to --"it's unreadable"[3] is extremely common, but questions abou[4] is a new one on me.
As it stands: I don't understand syntax highlighting. I'm pretty sure that syntax highlighting doesn't do anything for me in the best case. Maybe. But is it bad? Is turning it off necessary to being a better programmer? I got better when I turned it off, but it really might be a coincidence. I don't know.
[1]: http://kparc.com/b/a.c but not exactly.
[2]: https://news.ycombinator.com/item?id=8476294
I know I prefer a much more muted colour scheme now than I used to. I still like my syntax highlighting but it's more hinting than glaring RGB confetti.