My CSS snippet not working

Custom CSS & Theme Design
2

I figured out even though I have changed the suffix to .css, it’s still a RTF file.
I downloaded the css file provided below, and my snippet finally worked.

https://forum.obsidian.md/t/unable-to-get-any-snippet-to-work-in-obsidian-please-help/24509/6?u=shuoliu