if you are you using Harlowe 2, to change the background and text color there is no css needed:
Simply create a
header tagged passage (a passage with the tag 'header'), and include this in it:
(enchant: ?page, (text-colour: black) + (background: white))
For changing appearance of the whole story in general, I only found the hint to 'edit the stories stylesheet', which is reachable from the story menu of Twine2.
I changed 'body' to 'tw-story' of your css snippet, then background-color changed.
By the way, where did you get the info 'stylesheet tagged passages' from, I could not find this. Might be interesting, I only found the (css:) macro, which only changes the attached hook.