If you are writing CSS within <style> tag, that is not supported. CSS must be within style attribute of individual HTML tags and even then, only certain styles are supported.
Previously CSS was fully supported but it came at the cost. Back then Manager had to connect to 3rd-party server to convert HTML to PDF. This was slow and came with privacy concerns. The latest version is able to produce PDF files locally without connecting to 3rd-party server.
Why Manager can’t support entire CSS set? It could but that would mean to embed Chromium which is about 50 MB. In other words, Manager download size would go from 10 MB to 60 MB just to support this one marginal feature. It’s just not worth it.
So the long-term solution is to just keep improving internal PDF generator so it can support more of HTML/CSS subset as time goes.