Typically with a dark theme the text color would shift to something brighter, as shown in the screenshots where you can download the beta
If you open an existing project from MAC (and potentially any project) with the 64 bit installer for Windoes then the content in the manuscript doesn’t update colors. I never messed with fonts or colors, so this is just default behavior for Scrivener apparently. This doesn’t apply to the research folder or the Places folder.
See attached. Happens with all themes. Chose the blue one so you could at least see where the text would be.