- Drop a Label on Content
- Change its Text Fill color to say red in Inspector / Properties.
- Take "menubar > Preview > JavaFX Theme > Modena High Contrast - Yellow on Black" ==> on Content the rendering changes, yellow text on black background.
- Take again "menubar > Preview > JavaFX Theme > Modena High Contrast - Yellow on Black" to unset it ==> BUG on Content the text of the Label is black, the red color set by the user is not rendered (attached capture).

Note that if you do this scenario while Preview window is opened you'll notice the Preview window content remains correct.