While you cannot turn the entire program dark, you can customize the color. Go to File > New (or open an existing file). Click on the Stage (the white rectangle). In the Properties panel, find the Stage section.
Select a neutral dark gray or off-black background color ( #202020 ). adobe flash cs6 dark mode link
Look for active GitHub repositories or dedicated animation community forums (such as Newgrounds or specialized Discord servers) where users vet the source code. While you cannot turn the entire program dark,
This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later. In the Properties panel, find the Stage section
However, that does not mean there aren't solutions, but they require more effort, technical knowledge, and a degree of risk.