How to change text color in Formula Editor in Writer?

I use
openSUSE Tumbleweed KDE
GTK Theme: Breeze Dark
As you can see, the text does not change to white. Which setting will change this?

Build ID: 10(Build:2)
CPU threads: 8; OS: Linux 4.19; UI render: default; VCL: gtk3_kde5;
Locale: en-GB (en_GB.UTF-8); Calc: group threaded

I’m afraid you can’t. This window pane is just a “standard” editing window and I guess it uses OS defaults. I’ve searched through Tools>Options, LibreOffice>Application Colors but there is no section dedicated to Math. This is after all not surprising since Math is a neglected component, not having been updated for ages.

Eventually, try Tools>Options, LibreOffice>Personalization to import a theme or create your own. This may perhaps break the use of OS background and replace it with another.