clues on how to make the Save document pop-up more readable

When I Save from LibreOffice, a small “Save document?” dialog window pops up.
There are three buttons: “Don’t Save”, “Cancel”, and “Save”, which last is emphasised by a faint inner box, which barely shows that it’s the default selected choice.
Here is a screen grab.

In my Openbox installation, I have the active window bordered in orange, which can be seen, but I can’t figure how to improve the highlighting of the selected button choice within that dialog, and I can’t see any way to identify what kit LibreOffice’s VCL is using.

So I’ve tried:

SAL_USE_VCLPLUGIN=qt5; libreoffice
SAL_USE_VCLPLUGIN=gen; libreoffice
SAL_USE_VCLPLUGIN=gtk3; libreoffice

and none of these are any different in that Save dialog.

I’ve already successfully, in Openbox, fixed CopyQ’s tab buttons with Qt5 Configuration Utility
(Openbox Menu > Settings > Qt5 Settings > Appearance > Style > cleanlooks)

Any ideas how I can improve that LibreOffice dialog?