MainWindow.py (Código)

# -*- coding: utf-8 -*-
"""Utilizando name para aplicar um estilo personalizado (CSS)."""
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,
    )
@Gtk.Template(filename='MainWindow.ui')
class MainWindow(Gtk.ApplicationWindow):
    __gtype_name__ = 'MainWindow'
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
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/name-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)