Background colour settings of grc variable editor

Hello list,

using grc / gnuradio on Ubuntu (19.10 and 20.04) gives unreadable value
entries in the variable editor or in the parameter editor if the
parameter type is raw.
In such a case the background colur of the value entry is black and the
text colour is black, too. See attached screenshot.

Trying the same on a gentoo box everythings works fine.

So I suppose, it's first a problem of the linux distro, not of gnuradio.

But I could not find out wherefrom grc takes the settings of the value
entry in the variable / parameter editor.

Any hints would be appreciated

-- Volker