Created attachment 179119 [details]
Screenshot with and without area set to white
When you select a style on the sidebar, its background becomes Dark Blue and its name becomes White (well, in a typical desktop environment color scheme and not in "dark mode").
However, if you the set that paragraph style to have a light area color, say White, the area color does show, while the paragraph text remains White, as in a regular selected style. The result is a White-on-White item.
See also bug 137705 for dark themes. In a nutshell, the background of the Stylist should use the document background color and font color (accepting that in case both are white you wont see anything) and my first approach failed since the only Stylist tree took the color not the whole sidebar deck.
*** This bug has been marked as a duplicate of bug 115507 ***