It's funny how your question exactly duplicates GTK - Enable/set dark theme on a per-application basisGTK - Enable/set dark theme on a per-application basis, even your love to the dark theme is there. :)
To summarize, in gtk3 you use settings.ini file to change the default theme of a single application (this is per-user setting), while in gtk2 you used GTK2_RC_FILES environment variable pointing to appropriate gtkrc file.