Obsidian uses its own flavor of CSS, I think, in large part because of CodeMirror and Live Preview. Just dropping in a stylesheet from somewhere else isn’t going to work well unless it was made with Obsidian in mind.
Have a look here →
https://codemirror.net/examples/styling/
…and this post to get familiar with the Developer Console in Obsidian. I’d poke around in here to get an idea of how things are put together.