Where to change this colour setting in KDevelop?
Okay, found it. A friend told me, it's called a "tooltip" and can be changed under
System Settings > Application Appearance > Colors > Colors > Tooltip Background. This of course is troublesome, as the Tooltip Text is white by default.
So, I put it back to how it was in 11.04: Black text on light blue. The light blue has the value #BEDFF.