Sorry, just saw your question. Maybe you’ve found it out by now, but here I go anyways:
You create a textfile with the CSS code and give the file a title + .css (e.g.: Checkboxes.css).
You put that file into the snippets folder (Vault/.obsidian/snippets). If the snippets folder does not exist yet, simply create it.
Now you can activate your snippet in Obsidian under Settings -> Appearance -> CSS snippets.