For those of us who are used to setting syntax coloring in Visual Studio, Notepad++ does not automatically override the background color settings for all languages. You can run the Settings > Styler Configurator… command to change colors for each language.

To very quickly change background colors for all languages, edit these files:

"%UserProfile%\\Application Data\\Notepad++\\stylers.xml"

"%UserProfile%\\Application Data\\Notepad++\\userDefineLang.xml"

Run find-change operations to replace, say, bgColor="FFFFFF" with bgColor="C0C0C0".