CSS Styles not applied in GTK4 C language

Question

I am trying to add colors to the 2 buttons in gtk4 GUI written in c language. From the docs, I have found functions to load and set the css class name for 2 buttons. But for some reason the styles are not applied to my GUI, and I only get a window with a white background and white buttons.

#include <gtk/gtk.h>

static void on_active(GtkApplication *app){
    // Create a new window and grid
    GtkWidget *s_window = gtk_application_window_new (app);
    gtk_window_set_default_size (GTK_WINDOW (s_window), 800, 800);
    GtkWidget *s_grid   = gtk_grid_new();

    gtk_grid_set_column_spacing(GTK_GRID(s_grid), 15);
    gtk_grid_set_row_spacing(GTK_GRID(s_grid), 15);
    gtk_widget_set_halign (s_grid, GTK_ALIGN_CENTER);
    gtk_widget_set_valign (s_grid, GTK_ALIGN_CENTER);
    gtk_widget_set_hexpand (s_grid, TRUE);

    // Create provider for styling
    GtkCssProvider *cssProvider = gtk_css_provider_new();
    gtk_css_provider_load_from_file(cssProvider, g_file_new_for_path ("styles.css"));
    gtk_style_context_add_provider_for_display(gdk_display_get_default(), GTK_STYLE_PROVIDER(cssProvider), 
                                            GTK_STYLE_PROVIDER_PRIORITY_USER);
    gtk_widget_set_visible(GTK_WIDGET(s_window), TRUE);

    // Create label
    GtkWidget *label  = gtk_label_new("Hello World!");

    // Create buttons: PvP and PvAI
    GtkWidget *button1 = gtk_button_new_with_label ("1");
    gtk_widget_class_set_css_name(GTK_WIDGET_GET_CLASS(button1), "btn1");
    GtkWidget *button2 = gtk_button_new_with_label ("2");
    gtk_widget_class_set_css_name(GTK_WIDGET_GET_CLASS(button2), "btn2");

    // Set button size and position
    gtk_grid_attach(GTK_GRID(s_grid), label, 50, 0, 40, 10);
    gtk_grid_attach(GTK_GRID(s_grid), button1, 50,  50, 40, 10);   
    gtk_grid_attach(GTK_GRID(s_grid), button2, 50, 125, 40, 10);

    gtk_window_set_child (GTK_WINDOW (s_window), s_grid);
    gtk_window_present (GTK_WINDOW (s_window));
}

int main (int argc, char *argv[]) {
    // Create a new application
    GtkApplication *app = gtk_application_new ("com.example.GtkApplication", G_APPLICATION_FLAGS_NONE);
    g_signal_connect (app, "activate", G_CALLBACK (on_active), NULL);
    return g_application_run (G_APPLICATION (app), argc, argv);
}

here is the styles.css

btn1 {
  color: green;
}

btn2 {
  color: blueviolet;
}

Why isn't this code applying the css to the GUI application?

Answer 1

gtk_widget_class_set_css_name() is used when you are creating a new derived widget class. You can use gtk_widget_add_css_class(widget, "some-class") to add a new CSS class to an existing widget. And to refer the class you can use .some-class in your CSS.

The modified version of your code would look like this:

#include <gtk/gtk.h>

static void on_active(GtkApplication *app){
    // Create a new window and grid
    GtkWidget *s_window = gtk_application_window_new (app);
    gtk_window_set_default_size (GTK_WINDOW (s_window), 800, 800);
    GtkWidget *s_grid   = gtk_grid_new();

    gtk_grid_set_column_spacing(GTK_GRID(s_grid), 15);
    gtk_grid_set_row_spacing(GTK_GRID(s_grid), 15);
    gtk_widget_set_halign (s_grid, GTK_ALIGN_CENTER);
    gtk_widget_set_valign (s_grid, GTK_ALIGN_CENTER);
    gtk_widget_set_hexpand (s_grid, TRUE);

    // Create provider for styling
    GtkCssProvider *cssProvider = gtk_css_provider_new();
    gtk_css_provider_load_from_file(cssProvider, g_file_new_for_path ("styles.css"));
    gtk_style_context_add_provider_for_display(gdk_display_get_default(), GTK_STYLE_PROVIDER(cssProvider),
                                            GTK_STYLE_PROVIDER_PRIORITY_USER);
    gtk_widget_set_visible(GTK_WIDGET(s_window), TRUE);

    // Create label
    GtkWidget *label  = gtk_label_new("Hello World!");

    // Create buttons: PvP and PvAI
    GtkWidget *button1 = gtk_button_new_with_label ("1");
    gtk_widget_add_css_class (button1, "btn1");
    GtkWidget *button2 = gtk_button_new_with_label ("2");
    gtk_widget_add_css_class (button2, "btn2");

    // Set button size and position
    gtk_grid_attach(GTK_GRID(s_grid), label, 50, 0, 40, 10);
    gtk_grid_attach(GTK_GRID(s_grid), button1, 50,  50, 40, 10);
    gtk_grid_attach(GTK_GRID(s_grid), button2, 50, 125, 40, 10);

    gtk_window_set_child (GTK_WINDOW (s_window), s_grid);
    gtk_window_present (GTK_WINDOW (s_window));
}

int main (int argc, char *argv[]) {
    // Create a new application
    GtkApplication *app = gtk_application_new ("com.example.GtkApplication", G_APPLICATION_FLAGS_NONE);
    g_signal_connect (app, "activate", G_CALLBACK (on_active), NULL);
    return g_application_run (G_APPLICATION (app), argc, argv);
}

And the CSS file styles.css:

.btn1 {
  color: green;
}

.btn2 {
  color: blueviolet;
}