I noticed that some applications use the Adwaita Dark theme, while others use the light one. Is there a way to make all applications use the Dark one?
Replacing the main theme with the dark one in
This answer is indeed the ideal and "safe" (i.e no need to replace/mod any system files) method. However, currently neither this method nor Jeff's answer works for all apps. Coz only GNOME3/GTK3 has support for the
As GTK2 is deprecated and superseded by GTK3, all actively developed GTK2 apps are getting ported to or replaced by GTK3. Sooner or later, other toolkits like Qt, wxWidgets, etc may also implement support for GTK3. By that time, this method may become valid for all(maybe almost all) apps.
Simply replace the main theme with the dark one (in a special copy of Adwaita in your home dir):
Then use a tool like gnome-tweak-tool to set Darkwaita as your gtk theme, and the changes should take effect immediately.