One (or more) of these CSS variables should deal with that
[nik@gnomic ~]$ grep "bg_color" /usr/share/themes/Matcha-sea/gtk-3.0/gtk.css
@define-color theme_bg_color #f7f7f7;
@define-color theme_selected_bg_color #2eb398;
@define-color bg_color #f7f7f7;
@define-color selected_bg_color #2eb398;
@define-color insensitive_bg_color #fcfcfc;
@define-color theme_unfocused_bg_color #f7f7f7;
Or use GTK Inspector to check/modify your preferred theme
thunar -q && GTK_DEBUG=interactive thunar &