difficulty in understanding theme part of android client

I am having some confusion regarding theme part of android client . is The dark theme in android client designed separately or it is default? If it is separately designed then please tell me where can I find those files for modification

actually I want to change the theme of custom dialog boxes only in dark mode so what should be the approach, should I create separate theme file or just make changes in each dialog while programming.

please suggest if you have your own approach. thanks in advance