The lack of themability in KDevelope appears to be by design. This didn't bother me until I tried using 5.0.2. The first thing I noticed was that KDevelope is not distributed in a single "appimage" file. This makes the icons it uses by default in accessible for any sort of unsupported mod without rebuilding the whole thing. Unfortunately the chosen default icons are breeze themed. Which means gray scale toolbar icons. I don't use the breaze theme on my desktop and that's why. KDevelope should be made theme able to match the system theme regardless of what that is. Reproducible: Always
The AppImage ships a fixed icon set and theme and this is unlikely to change because of the implications on file size and complexity of having it changable. Your distribution packages, which you should prefer if available, use the platform theme's icons, colors and style, as they always did.