Use gtk_widget_add/remove_css_class() where possible

This makes the code simpler and more idiomatic to GTK 4.

src/gui/calendar-management/gcal-new-calendar-page.c

@@ -200,15 +200,12 @@ static void
 update_url_entry_state (GcalNewCalendarPage *self,
                         EntryState           state)
 {
-  GtkStyleContext *context;
-
   self->calendar_address_entry_state = state;
 
-  context = gtk_widget_get_style_context (GTK_WIDGET (self->calendar_address_entry));
   if (state == ENTRY_STATE_INVALID)
-    gtk_style_context_add_class (context, "error");
+    gtk_widget_add_css_class (GTK_WIDGET (self->calendar_address_entry), "error");
   else
-    gtk_style_context_remove_class (context, "error");
+    gtk_widget_remove_css_class (GTK_WIDGET (self->calendar_address_entry), "error");
 
   update_add_button (self);
   toggle_url_entry_pulsing (self, state == ENTRY_STATE_VALIDATING);

src/gui/event-editor/gcal-date-chooser-day.c

@@ -99,7 +99,7 @@ gcal_date_chooser_day_init (GcalDateChooserDay *self)
   gtk_widget_set_valign (widget, GTK_ALIGN_CENTER);
 
   gtk_widget_set_can_focus (widget, TRUE);
-  gtk_style_context_add_class (gtk_widget_get_style_context (widget), "day");
+  gtk_widget_add_css_class (widget, "day");
 
   self->label = gtk_label_new ("");
   gtk_widget_set_halign (self->label, GTK_ALIGN_CENTER);

src/gui/event-editor/gcal-date-chooser.c

@@ -562,7 +562,7 @@ gcal_date_chooser_init (GcalDateChooser *self)
                               "visible",
                               G_BINDING_SYNC_CREATE);
 
-      gtk_style_context_add_class (gtk_widget_get_style_context (self->cols[col]), "weekday");
+      gtk_widget_add_css_class (self->cols[col], "weekday");
 
       gtk_grid_attach (GTK_GRID (self->grid), self->cols[col], col, -1, 1, 1);
     }
@@ -577,7 +577,7 @@ gcal_date_chooser_init (GcalDateChooser *self)
                               "visible",
                               G_BINDING_SYNC_CREATE);
 
-      gtk_style_context_add_class (gtk_widget_get_style_context (self->rows[row]), "weeknum");
+      gtk_widget_add_css_class (self->rows[row], "weeknum");
       gtk_grid_attach (GTK_GRID (self->grid), self->rows[row], -1, row, 1, 1);
     }
 

src/gui/gcal-calendar-button.c

@@ -79,7 +79,7 @@ make_calendar_row (GcalCalendar *calendar)
   paintable = get_circle_paintable_from_color (color, 16);
   icon = gtk_image_new_from_paintable (paintable);
 
-  gtk_style_context_add_class (gtk_widget_get_style_context (icon), "calendar-color-image");
+  gtk_widget_add_css_class (icon, "calendar-color-image");
 
   /* source name label */
   label = gtk_label_new (gcal_calendar_get_name (calendar));

src/gui/gcal-event-widget.c

@@ -865,7 +865,7 @@ gcal_event_widget_init (GcalEventWidget *self)
 
   /* Starts with horizontal */
   self->orientation = GTK_ORIENTATION_HORIZONTAL;
-  gtk_style_context_add_class (gtk_widget_get_style_context (widget), "horizontal");
+  gtk_widget_add_css_class (widget, "horizontal");
 }
 
 GtkWidget*

src/gui/gcal-quick-add-popover.c

@@ -90,7 +90,7 @@ create_calendar_row (GcalManager  *manager,
 
   gtk_box_append (GTK_BOX (box), icon);
 
-  gtk_style_context_add_class (gtk_widget_get_style_context (icon), "calendar-color-image");
+  gtk_widget_add_css_class (icon, "calendar-color-image");
 
   /* Source name label */
   label = gtk_label_new (gcal_calendar_get_name (calendar));

src/gui/gcal-window.c

@@ -1076,12 +1076,7 @@ gcal_window_init (GcalWindow *self)
 
   /* devel styling */
   if (g_strcmp0 (PROFILE, "Devel") == 0)
-  {
-      GtkStyleContext *style_context;
-
-      style_context = gtk_widget_get_style_context (GTK_WIDGET (self));
-      gtk_style_context_add_class (style_context, "devel");
-  }
+    gtk_widget_add_css_class (GTK_WIDGET (self), "devel");
 
   gtk_widget_set_parent (self->quick_add_popover, GTK_WIDGET (self));
 }

src/gui/importer/gcal-import-file-row.c

@@ -88,7 +88,7 @@ add_grid_row (GcalImportFileRow *self,
                               "ellipsize", PANGO_ELLIPSIZE_END,
                               "max-width-chars", 40,
                               NULL);
-  gtk_style_context_add_class (gtk_widget_get_style_context (title_label), "dim-label");
+  gtk_widget_add_css_class (title_label, "dim-label");
   gtk_grid_attach (grid, title_label, 0, row, 1, 1);
 
   gtk_size_group_add_widget (self->title_sizegroup, title_label);

src/gui/views/gcal-month-cell.c

@@ -425,8 +425,6 @@ void
 gcal_month_cell_set_different_month (GcalMonthCell *self,
                                      gboolean       different)
 {
-  GtkStyleContext *context;
-
   g_return_if_fail (GCAL_IS_MONTH_CELL (self));
 
   if (self->different_month == different)
@@ -434,12 +432,10 @@ gcal_month_cell_set_different_month (GcalMonthCell *self,
 
   self->different_month = different;
 
-  context = gtk_widget_get_style_context (GTK_WIDGET (self));
-
   if (different)
-    gtk_style_context_add_class (context, "out-of-month");
+    gtk_widget_add_css_class (GTK_WIDGET (self), "out-of-month");
   else
-    gtk_style_context_remove_class (context, "out-of-month");
+    gtk_widget_remove_css_class (GTK_WIDGET (self), "out-of-month");
 }
 
 GcalContext*