You should be able to put any CSS in a snippet. If you need to alter the HTML, I think you’d need a plugin to do that (but I’m hardly an expert). If declaring a class on a whole note is good enough, you can use the cssclassmetadata field. You can assign classes some other was with plugins — search “class” in Settings > Community Plugins > Browse.
At this moment this is not possible. I asked the dev and she said it’s in the works but a far way off for now. To make use of the cyberpunk UI features I need to either change the HTML code of the obsidian UI or somehow sideload it via JS and attribute the data-augmented-ui through that to the