I do not think so, as I've always used CSS (https://developer-old.gnome.org/gtk3/stable/chap-css-overview.html) instead, via GtkCssProvider() (https://developer-old.gnome.org/gtk3/stable/GtkCssProvider.html) interface. It is exactly as simple, except that you have a second file (a .css one) along with the GtkBuilder .ui one.
In the simplest case, before you show your UI,
GFile *cssfile = g_file_new_for_uri("uri-to-css-file");
if (cssfile) {
GtkCssProvider *provider = gtk_css_provider_new();
if (!gtk_css_provider_load_from_file(provider, cssfile, NULL)) {
gtk_style_context_add_provider_for_screen(gdk_screen_get_default(), GTK_STYLE_PROVIDER(provider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
}
g_object_unref(cssfile);
}
where the URI can be e.g. resource://TLD/DOMAIN/APP/FILE.css, or you can drop the cssfile and use gtk_css_provider_load_from_path(provider, "path", NULL) (https://docs.gtk.org/gtk3/method.CssProvider.load_from_path.html) instead.
See GTK CSS Overview (https://developer-old.gnome.org/gtk3/stable/chap-css-overview.html) for how to target the count1 GtkEntry, or all GtkEntries.
For targeting only that particular GtkEntry, a simple
entry#count1 { font: 21px; }
should work.