They turn white for me (or green for sticky posts). However, there may be some combination of dark mode and theme setting that I've overlooked. The #app id should have the attribute data-bs-theme="dark" if you use darkly. If your entire system use dark mode it should also work by default if your system tells your browser that you prefer dark themes. (But data-bs-theme will still be set to light unless one go to the settings and change the theme to darkly.)
Aha, that was it - I forgot that I had set my Lemmy theme to litely and was relying on the Dark Reader FF extension to automatically switch between night and day with Gnome.
Switching Lemmy to darkly makes your theme display correctly. Now I just have to figure out a different way to get automatic day/night theme working properly 😅