As described in T12793 , this patch applies the KDE1-style 'Classik' titlebar icon styles to Breeze.
In this diff, for Breeze, it modifies both kdecoration and kstyle so that there will be consistency when titlebar-like buttons are used within applications.
In D28358 , for Breeze-GTK both new SVG files are added (affecting GTK applications with client-side-decorations in the titlebar like gedit), and the python script has been updated to generate new .png files with Cairo (affecting applications like Chromium).
In D28359 Breeze-Icons, the action icons for window operations have been updated to match the 'Classik' style.
Screenshot from before applying patch:
Screenshot from after applying patch:
Annotated version of previous screenshot, labelled in red with which modules are rendering which icons:
Another screenshot showing all the titlebar icons after the patch: