Once upon a time, GTK+ 3.0 was released. That release brought at least one Bad Thing™: incompatibility with GTK+ 2.x themes. At the same time, previously popular Clearlooks theme hasn't been ported. Many people didn't like that, but only one decided to DTRT — to do the Right Thing. Jean-Philippe Fleury wrote Clearlooks-Phénix (originally, Clearwaita), a GTK+ 3 theme which was supposed to have a look and feel as close as possible to the original Clearlooks. He based his work on an engine of a new GTK+ default, Adwaita theme. Quite soon, however, GTK+ 3 theme API has changed, and it became easily possible to rewrite the theme without using any additional theming engines, with just plain GTK+ stuff involved.
Then GTK+ 3.6 came and broke the API once more. Jean-Philippe fixed stuff to work with newer GTK+ again, and everyone was happy again.
But Empire striked back: GTK+ 3.8 brought more disruptive changes, rendering menus as ugly as never before. Unfortunately, Jean-Philippe was unable to cope with changes alone, he set up a mailing list to collectively develop Clearlooks-Phénix, but that didn't help, and no fix has been released during more than half a year.
As a maintainer and a user of Clearlooks-Phénix, I had to do something with this, as more and more things needed recent GTK+ 3 to work. After spending some time studying CSS code and playing with its knobs, I found a solution which proved to be quite simple, and uploaded an updated package. Unfortunately, the changes aren't in the upstream package yet.
Now, with GTK+ 3.10, stuff is not quite working again. Some changes in GTK+ internals lead to menu bars not being coloured properly. At the moment of this writing, an updated package is already available in unstable.