SUMMARY Switching from Breeze to Breeze Dark, everything related will be adjusted accordingly to provide a consistent look. However, the GNOME Application Style themes are omitted. STEPS TO REPRODUCE 1. Open System Settings. 2. Go to Workspace Theme. 3. Desktop Theme and switch to Breeze Dark. 4. Open Application Style > GNOME Application Style (GTK) OBSERVED RESULT * GTK-Designs still use Breeze * Prefer Dark GTK3 theme is not checked EXPECTED RESULT * GTK-Designs now use Breeze Dark * Prefer Dark GTK3 theme is checked SOFTWARE VERSIONS (available in About System) KDE Plasma Version: 5.13.5 KDE Frameworks Version: 5.50.0 Qt Version: 5.11.1 ADDITIONAL INFORMATION Same issue holds when switching from Breeze Dark to Breeze.
Nate Graham is also aware of this problem. For more information: https://pointieststick.wordpress.com/2018/09/29/this-week-in-usability-productivity-part-38/#comment-2042
We discovered that this setting had no effect whatsoever, so it has been removed in Plasma 5.18. So there's nothing to do here. GTK apps already change colors appropriately now as of Plasma 5.17.