I managed to partly solve the problem. It required deleting all old settings from "C:\Users\<username>\AppData\Roaming\PLSQL Developer 14" and "C:\Users\<username>\AppData\Roaming\PLSQL Developer 15" and starting completely fresh.
The text in question is now at least black and no longer light cyan.
However, there is still no option to set the default color for the editor text...