Yeah, I use light theme and didn't even notice any changes to the syntax coloration. I did immediately notice the font change after the update, but it doesn't give me a negative impression.
I think that Discourse is still playing. If I recall correctly the first iteration had bright yellow in it (dark theme) which I found painful. And in the topic that you linked there is a reply 14hrs ago (at the time of writing this) that inline code tags need to be fixed.
These are the changes that have been made to the Discourse forum framework between the time of the code block restyling ("UX: Codeblocks experiment merge") and the version currently in use by Arduino Forum:
I don't see anything relevant there.
It is possible that there was an adjustment to the custom theme used by Arduino Forum (the theme is maintained by the Discourse team, not by Arduino), but if I am interpreting the forum's administrative logs correctly, the theme has not been updated recently.
In case anyone is interested in learning how to determine which version of Discourse is currently in use by Arduino Forum:
Click here
You can determine the Discourse version in use by Arduino Forum by looking at the source of any forum page. There you will see something like this:
<meta name="generator" content="Discourse 3.4.0.beta3-dev - https://github.com/discourse/discourse version d71016522e8d9bb21c20312388271f8f0dd53069">