gtk-radiobutton-css

import gi
gi.require_version('Gtk', '3.0')
gi.require_version('Gdk', '3.0')
from gi.repository import Gtk, Gdk

class MyButton(Gtk.RadioButton):

   def __init__(self, *args):
        self.set_css_name("mybutton")
        Gtk.RadioButton.__init__(self, *args)


win = Gtk.Window()
win.connect("delete-event", Gtk.main_quit)

css_provider = Gtk.CssProvider()
css = b"""mybutton {
         border: solid 6px red;
       }
       """
css_provider.load_from_data(css)
screen = Gdk.Screen.get_default()
style_context = Gtk.StyleContext()
style_context.add_provider_for_screen(screen, css_provider, Gtk.STYLE_PROVIDER_PRIORITY_USER)

label = MyButton("Button 1")
win.add(label)


win.show_all()
Gtk.main()

Gtk.RadioButton and Gtk.ToggleButton