Ah OK, thank you!
I already thought you meant something like that - even though I have zero experience with the Themes unit, I did play with the test/demo application I found in the forum. Most controls do not seem to work under macOS though.
On that note; I guess I currently do something similar right now with BGRAControls (meaning: define the look and such).
The problem I'm running into is that regular controls, for example TButton, automatically adapt on the fly to theme changes. Latest SVN version work very well with macOS Dark Theme and Accent Colors and this seems to work pretty good under GTK2 as well. Note: But it doesn't work under Windows with it's new dark mode and accent colors, due to UWP vs Win32. The Themes demo/test shows that the Themes unit does not appear to respond to changes of themes either (under macOS).
Since I have no mechanism (by my knowledge) to receive a notification that a theme has changed, I wouldn't know when to change custom drawn controls (like your excellent suggestion, and BGRAControls) to match the changes (on the fly). Is there such a notification that I respond to?