What does it mean for me, application developer?
If your application matches the type of applications that would benefit from having a dark theme, please try it out. The 3 lines patch is easy to add to existing applications.
You'll most likely want to switch to using symbolic icons as well, for most of your UI, so that the icons show in the expected colour when a dark theme is not available.
What does it mean for me, GTK+ theme designer?
Either you keep your theme as-is. Then all applications will use that theme. This is most likely what should happen for Accessibility-related themes.
Or you create a "gtkrc-dark" file in your theme, at the same level as your gtkrc, and make it dark (not a bit darker like the bad example above, but real dark). Test it out with gtk-demo's “Application Window” demo.
By the way, you might want to update your theme for GTK+ 3.0 at the same time.