AFAICT the VTE docs don't specify any CSS selectors for the terminal, so this probably won't work (but not tested)
I assumed in the screenshot that VTE was taking the background colour from the GTK+ theme (and so therefore modifiable by CSS) but I realize now that the background colour was manually changed in the Preferences from Geany's defaults to be a colour that has low contrast with VTE's hardcoded default colour palette.