Hi Patrice

Thanks for the comment.
There are some images it cannot read the exif data of. But in my own experience it is does just as well or better than most other exif-reader addons. Also notice, even though a forum shows exif of uploaded photos, the exif data is sometimes stripped from the scaled or recompressed version actually shown on the forum pages.
The dark mode follows OS settings. In Windows 10 (requires latest Win10 1903) settings, you need to set Dark as your "Color" scheme, or Dark as "Default App mode" if using the "Custom" scheme:
Settings -> Personalisation -> Colors.
I hope that can help you a bit. Unfortunately I don't know how to set it up in Linux/Ubuntu, or if it is supported at all on Linux.
Will probably add a setting allowing you to force dark mode in final version.

UPDATE: Thanks a lot for your updated review :-)