The developers are working to eventually port the application to Qt
Audacious developers have decided that their software will go back to GTK2 from GTK3 due to some problems with the decisions made by the GTK+ makers, who are following a different road for their toolkit.Multiple developers have expressed their disagreement with the direction of the GTK+ project and it seems that Audacios devs are actually doing something about it. Ditching GTK3 is just a first step, because the ultimate goal is to switch to Qt.
“As anyone using Git master has noticed (or will notice soon), the GTK+ interfaces are now using GTK2 again rather than GTK3. The change was made because certain design choices in recent releases of GTK3 have made it increasingly difficult to write a ‘traditional’ PC / desktop program using the toolkit. The long-term goal is still to switch to Qt; however, the GTK+ interfaces need to remain stable in the meantime, and going back to GTK2 appears to be the only way to achieve this,” said John Lindgren in a forum post.
Some of the problems with the new GTK3 mentioned by the devs include monochrome icons, client-side decorations, and message windows that are mimicking Android.
In any case, the switch to GTK2 hasn’t hit the stable version and it will take some time before we see it in the regular application.