I installed about two dozens of plugins in a new Notepad++ installation and the result is:
I checked all plugins. None of them gives a hint to the background color. I also checked Settings → Preferences. To no avail.
The Change Markers and Location Navigate plugins mentioned in How to disable the "highlight changes" feature? aren't shown in my Plugins.
UPDATE
...\Notepad++\plugins:
_CustomizeToolbar
AutoSave
BetterMultiSelection
BigFiles
CodeAlignmentNpp
ColumnTools
Comment Wrap
ComparePlugin
ComparePlus
Config
CsvQuery
CustomLineNumbers
doc
HexEditor
HTMLTag
HugeFiles
ImgTag
IndentByFold
JSFunctionViewer
JSMinNPP
JsonTools
mimeTools
nppAutoDetectIndent
NppConverter
NppExport
NppJavaPlugin
NPPJSONViewer
NppXmlTreeviewPlugin
XMLTools
