OMNeT++ Installation Guide has a section for helping on fixing tooltip colors under Ubuntu using gnome-color-chooser.
Because Ubuntu Bionic 18.04 defaults to GTK-3 instead of GTK-2, this solution isn't working completely.
I also certified that all my themes under /usr/share/themes/ have these settings:
tooltip_fg_color:#
000000
tooltip_bg_color:#ffffff
But OMNeT (or Eclipse) kept not loading the right colors.
I hope it works for you too.
Good luck!