Welcome to the forum!
You don’t say which other posts you’ve used as reference. I went to this one Possible to customize the little Anvil spinny ball? - #7 and pasted their CSS into my Native Libraries section between style tags, and it just worked.
Then when I tried it using a file in Assets like you are with a relative reference, it stopped working.
When I updated the CSS to use an absolute reference (e.g. including the full https://etc domain before /_/theme/etc), it started working again.
So as a first step you might try updating your URL to be a full URL. If that doesn’t work, check the post I linked and see where your CSS is different.