Digikam is built without widget application style support. Since 5.2, Digikam uses a CMake switch to enable/disable application style support and it defaults to off on all platforms. Quoting README: Use Cmake "-DENABLE_APPSTYLES=on" flag to compile digiKam with widget application style support (disabled by default). Without this switch, Digikam defaults to Fusion style.
If this is a better setting it should be default. Moving to digikam.
Already reported as bug 372284. If that bug is rejected, please reopen it as a packaging bug.
According to bug 372284, Digikam isn't going to change the defaults. Please change the CMake options in Neon packages.
Created attachment 102761 [details] patch for debian/rules Just to confirm that this patch to the debian/rules from neon packaging git works properly, Digikam shows up using the system theme and theme selector is available in the settings.
Now that a new release of Digikam is up, could we have CMake updated for application style support?
yes it is, since a while : https://cgit.kde.org/digikam.git/tree/CMakeLists.txt#n105 Gilles Caulier
4:5.5.0-0neon+16.04+xenial+build24 has application style support enabled, thanks!