As of GTK+ 3.15.0, GTK+ relies on CSS instead of theming engines. I think Oxygen-gtk currently uses a theme engine to make everything work. This should be changed to CSS. Reference: http://blogs.gnome.org/mclasen/2014/10/23/an-early-view-of-gtk-3-16/ Quoting: "Pure CSS theming For the last few years, we’ve implemented more and more CSS functionality in the GTK+ style code. In 3.14, we were able to turn Adwaita into a pure CSS theme. Since CSS has clear semantics that don’t include ‘call out to arbitrary drawing code’, we are not loading and using theme engines anymore. We’ve landed this change early in 3.15 to give theme authors enough time to convert their themes to CSS." Reproducible: Always
WontFix, sorry. We do not have the manpower (nor the motivation) to dump all the code we write because gtk decides to drop half their theming API in between two minor releases. There is a bug report at gtk kindly asking them to reconsider. https://bugzilla.gnome.org/show_bug.cgi?id=735211 (I would personally consider dropping support for theming engine as an act of complete disrespect from gtk devs to the rest of the world)
*** Bug 342282 has been marked as a duplicate of this bug. ***
*** Bug 342318 has been marked as a duplicate of this bug. ***
I understand that it is not ok for gtk+ to drop functionality within gtk+ 3.x lifetime, But I feel that KDE users should be higher priority. If no one with the required knowledge is willing to port oxygen to gtk's css even at the expense of starting from scratch, it is the KDE users who lose and not the gtk+ developers. Sometimes in life, it is ok to swallow our pride and accept the loss of the countless hours of work put into coding oxygen-gtk in order to bring something better to KDE users who still use oxygen theme on KF5 ;) Oxygen was the face of KDE applications for long years. If anyone has the knowledge and skills to clone oxygen in gtk's css, please do :) (That or port firefox to qt5 :P )
*** Bug 356845 has been marked as a duplicate of this bug. ***