How to run gtk2 application with different theme from default chosen theme

I want to run my gtk2 application in dark style rather than white, so the command is:

env GTK2_RC_FILES=/path/to/gtkrcfile appname

for example,

env GTK2_RC_FILES=/usr/share/themes/Ubuntu\ Studio/gtk-2.0/gtkrc pidgin

will run pidgin using Ubuntu Studio dark style while my default theme is Ambiance. You can add the command in shortcut to ease launch the application.
(got it somewhere and I forgot where)