GTK applications tooltips are unreadable within Plasma 5

I’m experiencing a little issue with GTK apps inside Plasma 5.

The tooltips use a color scheme (white text over light grey backround) that makes them unreadable, as you ca see from the screenshot

https://i.imgur.com/lXl1sSr.png

How can I solve this?

Known problem.
https://bugs.kde.org/show_bug.cgi?id=355540

Try to use a different GTK theme, or maybe changing the setting “Apply colors to non-Qt applications” (in KDE’s settings->Colors) helps…