MainWindow.py (Código)

# -*- coding: utf-8 -*-
"""Gtk.CssProvider()."""
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'
    entry = Gtk.Template.Child(name='entry')
    label = Gtk.Template.Child(name='label')
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
    @Gtk.Template.Callback()
    def _on_button_clicked(self, button):
        if self.entry.get_text():
            self.label.set_label(str=self.entry.get_text())
        else:
            self.label.set_label(str='Digite algo no campo acima!')
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/custom.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)