I'm honestly not familiar with the user guide, but if elements indeed exist in the html with a class attribute value of "sigil_index_marker", then I would expect to see something like:
Code:
.sigil_index_marker {color:#335500};
in the css.
(provided the color is not being overridden somewhere else)
Though there's technically nothing wrong with the way the guide suggests (provided the index has been already been generated as Kevin mentioned.)