- Being eaten - This sounds like it’s probably a cloud issue. I suppose you have your vault stored in iCloud? Make sure you go to Preferences → Apple ID at the top → iCloud → Account Storage → Optimize Mac Storage → OFF!
If that is on, sometimes iCloud will offload your files into the cloud. And if you are using Obsidian Sync + iCloud, (or any combination of 2 or more sync tools, that is a recipe for disaster, because they can fight each other and duplicate or delete files. Please avoid that.)
- .rtf - You can simply rename the file, but you want to save it as plain text, not rich text. rtf is rich text format and has a bunch of extra stuff in it. In TextEdit, you have to switch to “Make Plain Text”, and then you can save it as .txt or .css (If you save it as something.txt, you can simply rename the file to something.css and it will work. You cannot do this from something.rtf.)
- Finding the snippets folder - I’m confused because your screenshot is showing the snippets folder. If you are in your vault folder, and you hit Cmd-Shift-. it should show hidden files, and you should see your .obsidian settings folder. But it looks like you’re already there.
(I can’t help with the CSS part at the moment.)