But in C# and WinForms it's pretty easy to change the colours of a control. In webtech-based desktop applications, it's not only easy, it's amazing. And there's probably loads of other toolkits that can do custom colouring.
If your toolkit can't do this, maybe it's beginning to be time to be looking at something else after all these years. Not just for darkmode, but for any kind of drawing that makes the program a little bit easier on the eyes. Because let's be honest, TC is not exactly a super appealing 2019-era-looking program anymore.
And as further encouragement: the Android version is totally darkmode. Brilliant

Totally different thing, I know, but just saying because it's still also TC.