Ah I see, I missed what @jamieQ had posted was a before shot not another now example.
I believe this is indeed just an update to Discourse, not a choice by the project. We can look into how customizable per site it is (I assume it is) or per user (less likely, perhaps).
I must say, though, that looking at them side by side, the new coloring seems like an improvement to my eyes. I can understand concern the colors are garish. However, the old colors were very subdued – only using bold for keywords, not a color (which is very subtle difference) and using the same color for comments as code is fairly unusual in IDE color schemes. The new one is more in keeping with the default coloring of both Xcode and VS Code.
I can see the argument the new colors are too much, but also that the old colors were too little. Personally, I do think is that the darker background on code blocks is an improvement, as is differentiating code from comments IMO. And I also happen to prefer not using bold for certain highlighting, as I find it makes fonts less readable.