seems "define one colour, define them all" is not done with the raspi forum.
I have a XFCe white on dark theme ("Nodoka-Midnight"), with near dark background and almost white light grey text.
Now, when writing a post in here, the Subject and Edit boxes use my themes background colour, but you define the text colours as black as it seems...
You should also define all other colours, since folks like me with light text on dark background themes have a hard time writing posts:
Same goes for the search edit box at the bottom of a page...
Cheers