it’s nothing inherent to v8.4.2.

Which theme have you lost? Have you looked in %AppData%\Notepad++\themes (which is the default location; see also Cloud and -settingsDir if one of those options is used). Have you looked in c:\program files\Notepad++\themes or wherever you installed Notepad++? (Note that if you had shared ?-menu’s Debug Info, I would have been able to narrow them down to precise paths, because all those options would have been shown to us)

If it’s one of the standard themes, you could try re-installing Notepad++ , or grabbing the portable zip and extracting the right theme config file and placing it in the appropriate Themes folder for your installed Notepad++