05-10-2018, 03:23 PM
Using a Kubuntu 18.04 VM (so as not to screw up my 16.04), try as I might, not able to replicate your screenshots.
It will be in KDE System Settings somewhere. Obviously you like dark themes, but try something else maybe a default Breeze with a Desktop Breeze light.
Your could also look in Applications Style and GNOME Application Style (GTK) settings.
If you give what theme/desktop/color settings you use, might lead to a solution. I do not think the problem is Gimp, more incompatible KDE options.
It will be in KDE System Settings somewhere. Obviously you like dark themes, but try something else maybe a default Breeze with a Desktop Breeze light.
Your could also look in Applications Style and GNOME Application Style (GTK) settings.
If you give what theme/desktop/color settings you use, might lead to a solution. I do not think the problem is Gimp, more incompatible KDE options.