Changing style of Emacs' tooltips

It seems obvious now that I figured it out.

Some parts of Emacs GUI are not configurable through faces (obvious). My Emacs is compiled with GTK 3.0, so it uses that library to render some of the elements of the GUI (even more obvious). Tooltips are among those elements. So, in order to change their style, I need to intervene on the configuration of GTK. For example, I had this:

Creating a file ~/.config/gtk-3.0/gtk.css:

@define-color tooltip_bg_color #fff8dc;
@define-color tooltip_fg_color Black;

and restarting Emacs, I obtain this: