In System Settings/Appearance/Colours, down the bottom, find “Apply colours to non-QT applications”.
Uncheck it.
Gimp tooltips should now show white on black.
There are no downsides that I have encountered so far.
Thanks. I also could not read the tooltips and did not know why.
Maybe we can add that this is when one runs Gimp in KDE/Plasma and that those Systemsettings are the KDE/Plasma ones (from the main menu) and not from Gimp itself.