The last few days there was a complete refactoring of Adwaita theme with the porting to SASS. The goals for this, was firstly to make the theme easier to maintain, but most importantly, the theme rewrite is aiming to clear the logic of the the custom stylesheets for individual applications.
Together with those, we have many changes in visuals, new classes that permits us to style more GTK+ elements, and the transfer of some styles from GTK+ core to GTK theme, making theming even more flexible.[caption id="attachment_26817" align="aligncenter" width="640"] Adwaita 3.14 already has dozens of smaller of bigger changes![/caption]
Today Matthias Clasen brought two more significant changes. For start, Adwaita managed to get rid of the theme engine, and secondly, Adwaita became the default theme of GTK+ (previous was Raleigh) and it is now part of LibGTK.
This last change was made mostly for portability reasons, so GTK Apps can look the same in every platform, Linux, Windows, OS X, with the option to can change it.
Concerning the development of Adwaita theme, my guess (not sure yet) is that they will keep “gnome-themes-standard” repository, for easier maintainability and they will just push the changes to GTK+.[caption id="attachment_26818" align="aligncenter" width="640"] The embedded in GTK+ 3.14 Inspector will make theme authors life lots easier![/caption]
If you find the current (3.12) Adwaita theme appealing, you will get a nice surprise with what you will see in 3.14. The “blaming” goes to Jakub Steiner and Lapo Calamandrei which are the main developers of Adwaita 3.14 so far.
Update: Matthias Clasen wrote about it at: A NEW DEFAULT THEME FOR GTK+