Dark mode and other theming options for PSPP

[Mukherjee, Shivam]

Greetings everyone!

I have used SPSS for helping my friends with their university statistics work. 
I downloaded PSPP a few hours ago and immediately felt that it could do with 
customisability for (at the minimum) switching to dark mode, if not tracking 
the system's preference.

I suppose the easiest thing to implement would be to produce some changes to 
the GTK specification, or even just make new dark-themed Adwaita icons.

What could I do to achieve this? I'll want to explore the source code some time.

Regards,
Vivraan.