@metal3d There's no such thing as a "default color". Those colors are the ones currently used by libadwaita, if you use the names defined by libadwaita. Otherwise you need to know the names defined by the Default theme; the Default GTK theme is not API. Of course, any other theme can define its own colors.
At this point, I have *no idea* what you're expecting. Instead of using Masto, you may want to use Discourse: https://discourse.gnome.org/tag/gtk