I highly recommend reading this blog post to understand the vocabulary here.
I love Plasma’s appearence settings for the fact that they support QtWidgets, QML and GTK, and apply to apps even inside of Flatpaks. I have found it to be necessary for me considering that default settings are sometimes not usable (especially with QML apps). With Breeze you can even have a consistent style accross Qt and GTK, since the Breeze stylesheet for GTK supports KDE’s color schemes.
On the Qt side, settings are applied through the KDE platform plugin, which is somehow automatically used under KDE. On the GTK side, as far as I am aware, systemsettings applies them and they stay applied as long as they’re not overridden.
Other DEs I used did not include all of the above functionality. Standalone window manager setups simply lack those. qt5ct is a popular option for Qt but it does not handle QML nor KColorScheme.
To have the same features within a standalone WM, I figured I’d simply use KDE’s systemsettings and force the use of the KDE platform theme on Qt apps. This is achieved by having systemsettings and plasma-integration installed, and setting the environment variable QT_QPA_PLATFORMTHEME to kde under your WM setup. Then you can launch systemsettings (from a terminal or run dialog if it doesn’t appear in your menu) and adjust the appearance settings as you’d like.
This is a stupid workaround but it works great. Please do tell me if this should be avoided or if you have a better solution.