How to override GTK style setting from kde-config-gtk-style for one specific app?

I am quite happy, how my kde on debian handles the theming for gtk apps like firefox or thunderbird. I usually use the breeze-dark or adwaita-dark themes. However, I use one application (moneyplex) that never looks good with my gtk theming. There are text fields that have ugly white backgrounds with light grey font color in them, where their background should be dark. So I really want to set an override theme for just that one application. I found a frequently named solution to try something like GTK_THEME=Adwaita:light moneyplex and variants of it but it does not work at all. Is there any other environment setting that will override the current gtk theme engine for one app?