@WillAdams, yes, I have been doing that since Win95 days. That is what I'm talking about when I say the Windows themes only extend to the window borders and headers (and a select few MS accessories like Notepad).
SolidWorks has extensive color control over its in-window elements
Gimp has selectable (and I assume editable, but I haven't tried) themes.
But both of those application windows borders/heading have my current Windows UI settings, which are common to all applications, even my now-12-year-old VersaCAD for Windows, which I've been using since XP days (and its pure-DOS ancester since 1989, which ran natively up through Win98SE) and retains its DOS-based in-window interface...