How to style a GtkLabel with CSS?

I'm trying to use CSS to style a GtkLabel. I would like to change the color and font size of the label. Here is my C code:

#include <gtk/gtk.h>

int main(int argc, char *argv[]) {
    gtk_init(&argc, &argv);

    GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    GtkWidget *label = gtk_label_new("Label");

    GtkCssProvider *cssProvider = gtk_css_provider_new();
    gtk_css_provider_load_from_path(cssProvider, "theme.css", NULL);
    gtk_style_context_add_provider(gtk_widget_get_style_context(window),
                                   GTK_STYLE_PROVIDER(cssProvider),
                                   GTK_STYLE_PROVIDER_PRIORITY_USER);

    g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
    gtk_container_add(GTK_CONTAINER(window), label);
    gtk_widget_show_all(window);

    gtk_main();
}

Here is my CSS in theme.css, currently this should only change the font color:

GtkLabel {
    color: green;
}

I compile with:

gcc `pkg-config gtk+-3.0 --cflags` test.c -o test `pkg-config --libs gtk+-3.0`

However, when I run the code I get black text. If I change theme.css to use a widget name or CSS class instead of GtkLabel, it still doesn't work. However, if I use * instead of GtkLabel, it works (although I don't want this to apply to everything). This means that the C code should be correct and something is wrong with the CSS.

Currently, CSS providers are not inherited to the children style contexts. So you need to add the CSS provider to the screen using gtk_style_context_add_provider_for_screen()

Try changing

gtk_style_context_add_provider(gtk_widget_get_style_context(window), GTK_STYLE_PROVIDER(cssProvider), GTK_STYLE_PROVIDER_PRIORITY_USER);

gtk_style_context_add_provider_for_screen(gdk_screen_get_default(), GTK_STYLE_PROVIDER(cssProvider), GTK_STYLE_PROVIDER_PRIORITY_USER);

I don't think gtk supports multiple screens these days, but gtk_widget_get_screen() could be used to replace gdk_screen_get_default().