If you look at the passage HTML elements generate by Harlowe you will see it has a structure something like the following:
(note: I am excluding elements that are not relevant to this topic)
<html>
<body>
<tw-story>
<tw-passage>
<tw-sidebar>
The contents of the passage being displayed.
Notice that the tw-story element (and it's decedent elements (tw-passage, tw-sidebar, etc)) is at the same level as the body element, which is different to most HTML web pages.
This is why the font size you are assigning to the body element is not effecting the story passage being shown, to fix your CSS change it to the tw-story element:
Comments
(note: I am excluding elements that are not relevant to this topic) Notice that the tw-story element (and it's decedent elements (tw-passage, tw-sidebar, etc)) is at the same level as the body element, which is different to most HTML web pages.
This is why the font size you are assigning to the body element is not effecting the story passage being shown, to fix your CSS change it to the tw-story element: