To clarify a few things.
Preferences -> Markdown -> Enable table of contents extension (wysiwyg: no)
No CSS is necessary to display a table of contents in the rendered view of a markdown note. Just add
a [toc] (at the beginning of a line and that's it.
However, if you want the toc to look different as to what you get, you will have to use CSS.
Here's a great topic about CSS:
This might also be of interest: