On the top is a QT toolbar as styled by Breeze-QT. The activated tools are highlighted with gray boxes. On the bottom is a GTK headerbar as styled by Breeze-GTK. The activated tools are highlighted with harsh highlight-color boxes. Making them gray in Gnome headerbars would be more consistent, predictable (some icons don't invert), and makes for a less distracting interface. Should be a simple fix, but would go a long way.
Created attachment 119682 [details] toolbar vs headerbar
Indeed it should. It's all CSS; would you like to have a go? https://cgit.kde.org/breeze-gtk.git/tree/src/gtk320/widgets/_button.scss
Fixed along with the fix for Bug 356006.