I also have notebooks like that. I want to implement a configuration option, which allows people to do this. Hopefully this will be included in the next version. In the meantime, if you are happy to edit the source, it is quite a easy change. Find the file spyder_notebook/utils/servermanager.py . Line 124 should read:
self.dark_theme = dark_theme
Change this to
self.dark_theme = False
Your notebooks will now always be displayed with a white background.
Jitse