MainWindow.py (Código)

# -*- coding: utf-8 -*-
"""get_style_context().add_class()."""
import gi
gi.require_version(namespace='Gtk', version='3.0')
from gi.repository import Gio, Gtk, Gdk
def load_custom_css(file):
    css_provider = Gtk.CssProvider.new()
    css_provider.load_from_path(path=file)
    screen = Gdk.Screen()
    style_context = Gtk.StyleContext.new()
    style_context.add_provider_for_screen(
        screen=screen.get_default(),
        provider=css_provider,
        priority=Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION,
    )
class MainWindow(Gtk.ApplicationWindow):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.set_title(title='get_style_context().add_class()')
        self.set_default_size(width=1366 / 2, height=768 / 2)
        self.set_position(position=Gtk.WindowPosition.CENTER)
        self.set_default_icon_from_file(filename='../../assets/icons/icon.png')
        vbox = Gtk.Box.new(orientation=Gtk.Orientation.VERTICAL, spacing=6)
        vbox.set_border_width(border_width=12)
        self.add(widget=vbox)
        for i in range(1, 3):
            label = Gtk.Label.new(str=f'Label {i} COM classe label-bg-red')
            label.get_style_context().add_class(class_name='label-bg-red')
            vbox.pack_start(child=label, expand=True, fill=True, padding=0)
        for i in range(1, 3):
            label = Gtk.Label.new(str=f'Label {i} SEM classe label-bg-red')
            vbox.pack_start(child=label, expand=True, fill=True, padding=0)
        self.show_all()
class Application(Gtk.Application):
    def __init__(self):
        super().__init__(application_id='br.natorsc.Exemplo',
                         flags=Gio.ApplicationFlags.FLAGS_NONE)
    def do_startup(self):
        Gtk.Application.do_startup(self)
        # Carregando e aplicando o arquivo de css personalizado.
        load_custom_css(file='../../data/css/class-label-bg-red.css')
    def do_activate(self):
        win = self.props.active_window
        if not win:
            win = MainWindow(application=self)
        win.present()
    def do_shutdown(self):
        Gtk.Application.do_shutdown(self)
if __name__ == '__main__':
    import sys
    app = Application()
    app.run(sys.argv)