This override is still needed until Qt minimum version requirement is
increased to at least 6.3.0.
ui-config.h.in and CMake 2 are modified to avoid USE_XDG redefinition in
CMake 3.
The system theme was named Default even though the default theme is Dark.
This addresses that by renaming Default.qss to System.qss. I've made it
backwards compatible so users already using this theme are not affected.
The theme list now shows up as:
-System
-Dark (Default)
-Acri
-Rachni
I have also made it so that you can specify the default theme in the
UI config file.