There isn't a dark theme mode. The only way is to change your window colors and such. But in Windows 10 this support is legacy, and very wonky. For example my PC randomly resets to my old dark themes i set up for someone else, which doesn't even work great in TG cause the text can't be changed that I can find. So then you can't read stuff well. In Windows 7 or XP this should work better by just changing window and button and text colors.