Hmm, if you want to use Qt5 (I do!) then its very easy to play with colours using qt5ct. You choose the colors you want to use and then set a env var, QT_QPA_PLATFORMTHEME=qt5ct and start your app.
qt5ct comes with a small set of colour themes, but dead easy to copy one and edit its colours, watch your app change (or squirm) as you play. IDK is qt5ct is available on Windows, I use native Windows there.
Another approach, on Linux at least is to make your qt5 app follow the existing selected theme. Again, an env var, this time QT_QPA_PLATFORMTHEME=gtk2 and it matches your other (ie gtk2/3) apps. You might need to install qt5-style-plugins.
Further, on qt5, you can play with eg Buttons color property, don't try that on gtk2 by the way.
Davo