OK, here are more test results about this bug.
I used the last Ubuntu version (17.10 Artful Aadvark) with a fresh install and the default Gnome desktop. The Qt version is 5.9.1. No custom themes installed.
1. Using gnome-tweak-tool, I switched the default Ambiance theme to Adwaita-dark. This theme is a standard Gnome GTK3 theme, provided with Adwaita as an alternative to the default theme. I used the dark variant because it allows to immediately see the theme differences between versions 0.87 and 0.88 of Xnview. The dark variant is standard as is the light one.
2. Screenshot xnview-0.87-adwaita-dark.png shows the look of Xnview version 0.87. As you can see, the background is black, as desired. The system theme Adwaita-dark is used by Xnview.
3. Screenshot xnview-0.88-adwaita-dark.png shows the look of Xnview version 0.88. Now, the background is light grey. The system theme is not used by Xnview.
4. I did some research on the Internet and found that starting with Qt 5.6, the system GTK theme can't be used directly, as it was the case with previous versions (indeed in Ubuntu 16.04, I used various Qt applications without any theme problem). There seems to be no global solution to this problem, except for the Adwaita theme. For this particular theme indeed, one has to install the package adwaita-qt and call Qt 5 applications with the '-style' parameter and then the Adwaita (or Adwaita-dark) theme is used by the Qt 5 application.
So I tried with Xnview 0.88 with the adwaita-qt package installed and running Xnview using the command: 'xnview -style Adwaita-dark'.
The result was the same as before, i.e. light background, dark theme not used.
On the console, Xnview returned this message (among others):
'QApplication: invalid style override passed, ignoring it.'
Then I tried with the Speedcrunch application, a scientific calculator written with Qt 5. I launched 'speedcrunch' and the dark theme was not used. I launched 'speedcrunch -style Adwaita-dark' and the dark theme was used.
I also tried with VirtualBox 5.2, which is another Qt 5 commercial application that recently switched to Qt 5.9. I launched VirtualBox without the '-style' parameter and the dark theme was not used. Then I launched 'VitualBox -style Adwaita-dark' and the dark style was used, as expected.
5. I also tried to launch the XnView binary, directly from the /opt/XnView directory, with and without the '-style' option and in both cases it crashed with the following error message:
'This application failed to start because it could not find or load the Qt platform plugin "xcb" in "".'
So clearly, Xnview 0.88 can't use the '-style' parameter and always use the default Fusion Qt theme. Unfortunately, I don't have any idea about what causes the problem but I can perform other tests if you need it.