When CSS is bundled into one file, it 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 Flarum. Right now Flarum's CSS is quite small and simple, so this is a great time to easily implement this separation and not suffer in the future with separation of much large and complex CSS when Flarum will become more mature.