Currently you can set options for Qt apps and GTK apps separately, but it would make much more sense to set the options for GTK apps in the respective KCMs too, instead of separately in the GTK KCM.
Here are the individual work items for this task:
[x] Fonts: D24743 (fixes https://bugs.kde.org/show_bug.cgi?id=138866)
[x] Icons: D24743
[x] Cursor theme D24743 (fixes https://bugs.kde.org/show_bug.cgi?id=401507)
[x] App color theme: D22877 (fixes https://bugs.kde.org/show_bug.cgi?id=400101, https://bugs.kde.org/show_bug.cgi?id=356006, and https://bugs.kde.org/show_bug.cgi?id=357524)
[x] Scrollbar click settings D25703
[x] Toolbar, menu and button icons visibility/style D24743
[z] Window decorations order D25670
[ ] Window decorations appearance (fixes https://bugs.kde.org/show_bug.cgi?id=414113)
[x] GTK2/3 themes D26583
[x] Dark theme preference D25945