Thanks, this is clearly suboptimal and unintended.

When the gnucash theme colors are used the text color (and probably the text selection color as well) should also be set to match the green/yellow theme.

While waiting for a fix, there *is* a way to override colors, but unfortunately it's not documented yet:
You can add a snippet of css code in
$HOME/.gtk-3.0-gnucash.css
such as this:

*.register-header {
  color: black;
}

*.register-primary {
  color: black;
}

*.register-primary:insensitive {
  color: black;
}

*.register-secondary {
  color: black;
}

*.register-secondary:insensitive {
  color: grey;
}

*.register-split {
  color: black;
}

*.register-split:insensitive {
  color: grey;
}

*.register-cursor {
  color: black;
}

*.register-cursor:insensitive {
  color: grey;
}

As a sidenote to the developers, the location $HOME/.gtk-3.0-gnucash.css should be fixed to $HOME/.config/gtk-3.0-gnucash.css. Preferably before the gnucash 3.0 release still!

The unreadable dark theme fix has been merged into unstable. The issue of
the config file location needs some more work. I'll comment on the PR.

And meanwhile the location of the config file has been settled as well.
As of gnucash 2.7.5 it will be
- $HOME/.config/gnucash/gtk-3.0.css (linux and other platforms following freedesktop specs)
- CSIDL_APPDATA/Gnucash/gtk-3.0.css (Windows)
- $HOME/Application Support/Gnucash/gtk-3.0.css

Hmm, that should probably be $HOME/Library/Application Support/Gnucash/gtk-3.0.css instead...

GnuCash bug tracking has moved to a new Bugzilla host. This bug has been copied to https://bugs.gnucash.org/show_bug.cgi?id=792809. Please update any external references or bookmarks.