When CSS is bundled into one file, even after tree-shaking, it still contains a lot of unused CSS because of different @media contexts (mobile, tablet, desktop, dark/light/other themes, etc.).
It’s bad for Core Web Vitals. Splitting of CSS by @media conditions gives huge boost in performance.
More info on this:
Please consider implementing of this optimization in Discourse. Currently this problem is the thing which prevents me from switching to Discourse.
Please note what Reduce unused CSS and Eliminate render-blocking resources(which are all CSS in your case) are TOP 1 and TOP 2 recommendations for Discourse fourms by pagespeed insights.
Thank you for your amazing job!