Issue with default Unity styling

issue_sample.py

from gi.repository import Gtk

builder = Gtk.Builder()
builder.add_from_string(
"""
	<?xml version="1.0" encoding="UTF-8"?>
	<interface>
	  <object class="GtkWindow" id="window1">
	    <property name="can_focus">False</property>
	    <child>
	      <object class="GtkFixed" id="fixed1">
	        <property name="visible">True</property>
	        <property name="can_focus">False</property>
	       	<property name="width_request">500</property>
            <property name="height_request">500</property>
	        <child>
	          <object class="GtkButton" id="sample_button">
	            <property name="label" translatable="yes">button</property>
	            <property name="width_request">250</property>
	            <property name="height_request">166</property>
	            <property name="visible">True</property>
	            <property name="can_focus">True</property>
	            <property name="receives_default">True</property>
	            <style>
	              <class name="sample_button"/>
	            </style>
	          </object>
	          <packing>
	            <property name="x">146</property>
	            <property name="y">114</property>
	          </packing>
	        </child>
	      </object>
	    </child>
	  </object>
	</interface>
"""
) # very simple sample of glade theme
window = builder.get_object("window1")

styler = Gtk.CssProvider()
styler.load_from_data(
"""
GtkButton {
background: url('http://24.media.tumblr.com/tumblr_m8v8njYAeb1rxukhwo2_500.jpg');
border: 0; color: red;
}
GtkButton:hover { color: green;}
"""
)
button = builder.get_object("sample_button")
button_context = button.get_style_context()
button_context.add_provider(styler, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)

window.connect('delete-event', Gtk.main_quit)
window.show()
Gtk.main()