I think so long as it's controlled by the user viewing the page and not the person posting, it's a fine and dandy idea.
As it is I've got a custom user.css I use to make it all one colour, because I find colour syntax highlighting to be a useless, blurry, illegible train wreck acid trip that makes it very hard for me to tell what the code even says. Some colour combinations of course being worse than others for this -- many of the popular ones not even coming close to accessibility minimums -- but by the time you get to legible contrasts, you can barely even tell there's colour anymore defeating the purpose.
I've watched people make mistakes where they couldn't even read their own code because of the fancy colours -- I often wonder if it's that failing that makes people reliant on automation like auto-completion that I find equally annoying.
It was ugly and useless 30 years ago when I first encountered it, it's only gotten more obtuse and annoying since that time. If I wanted to taste the rainbow that bad, I'd pop a dot.
In case you couldn't guess, not a fan.