I’d like to add a corollary, here.
It may not be obvious that items in the config.toml MUST be added in the same order as they appear in the documentation. It makes sense once you realize how TOML works, so it’s not technically a bug but probably this can be better explicitedly in the tutorial (which should not assume too much from the reader).
I see a couple of threads in the forum from people tripping over this detail. In my case was not loaded because by mistake the theme="" directive was after the [markdown] section.
WRONG
[markdown]
highlight_code = true
....
theme="after-dark"
RIGHT
theme="after-dark"
[markdown]
highlight_code = true
....