import gi
gi.require_version("Gtk", "3.0")

from gi.repository import Gtk, GLib, Gdk

style_provider = Gtk.CssProvider()
style_provider.load_from_data("""
    * {
        border: 1px solid black;
    }
""")
Gtk.StyleContext.add_provider_for_screen(
    Gdk.Screen.get_default(),
    style_provider,
    Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION
)

w = Gtk.Window(default_height=300, default_width=300)

frame = Gtk.Frame()
frame.set_label("#################")
frame.set_shadow_type(Gtk.ShadowType.NONE)

def draw(widget, ctx):
    ctx.set_source_rgb (0.7, 0.9, 0);
    ctx.paint()

content = Gtk.Box()
content.connect("draw", draw)
frame.add(content)

w.add(frame)
w.show_all()
GLib.unix_signal_add(0, 2, Gtk.main_quit)
Gtk.main()
