You do have access to your blog's CSS on GitHub if you'd like to make changes. The only files that you should avoid modifying on your own are the CNAME, _config, and any of the blog post files.
If you do edit a blog post directly on GitHub, you will see the changes on your live blog. However, the changes will not be synced to Learn. If you subsequently modify the post through the Learn.co blog editor, any changes you made to the post directly on GitHub will be overwritten with the new version of your post.