Since 5.2, Digikam uses a CMake switch to enable/disable application style support and it defaults to off on all platforms, not only Windows/OSX, as per bug 365262. Quoting README: Use Cmake "-DENABLE_APPSTYLES=on" flag to compile digiKam with widget application style support (disabled by default). The default value of application style being disabled, makes Digikam use the Fusion theme even on Digikam's primary platform, Linux/KDE/Plasma.
And what's the problem. Switch the flag to on in this case... Gilles Caulier
why is the flag not on by default if this is intended?
Because we need to homogeneous everywhere : Linux but also Windows, and MacOS. Under Windows and MAcOS, Fusion style is the only one which give good result. All others one are horrible and introduce dysfunction. So if Linux packagers want to use more style, they can, through the compilation option. But that all. Gilles Caulier