So, not too sure of my way around Windows. In Linux, there is a tool,
qt5ct (or qt6ct) that gives you very simple, very easy and complete control of how you app works.
I cannot find any mention of a windows version but maybe a more committed windows user can do better ?
All it does is establish a small set of config files with your choice of colours, you start your app with a particular env var that tells the Qt app to look at those config files. As most of that behaviour is built into Qt5 itself, I'd expect it to be there in a Windows Qt binary as well.
Is it worthwhile investigating to see if I am right, Windows Qt apps also obey that env var ? It would be dead easy to make a Lazarus app that writes the config files ....
Easy way to find out if your Windows Qt5 binary knows about the env var is to search the binary for it, here are two ways I would do it under Linux, maybe a windows user will recognise one or the other and apply the same trick to your Lazarus Qt5 binary -
dbannon@dell:~$ strings tomboy-ng-qt5 | grep QT_QPA_PLATFORMTHEME
QT_QPA_PLATFORMTHEME :
QT_QPA_PLATFORMTHEME
QT_QPA_PLATFORMTHEME
dbannon@dell:~$ grep -ni QT_QPA_PLATFORMTHEME tomboy-ng-qt5
Binary file tomboy-ng-qt5 matches
Or put that lazarus windows qt5 binary somewhere I can find it and I will search it for you ......
Davo