Are you talking about the white background color in the whole editing window?
If so, and you really didin't find anythink in the options menu, there just might be two or three additional possibilies:
- Calibrate your monitor.
- Add a new pattern to Options -> Syntax Highlighting -> filetype -> (probably) HTML, called background with a search pattern for *everything* (maybe .? will work - I'm not good at perl regular expressions) and set the background color.
- Get a GTK+ documentation about the editing widget and find out if it has a method for setting the background. Then tweak the sources and recompile.