Add the "devel" CSS class for development builds

The main window should be styled to reflect that it is a development
build.

Helps: endlessm/endless-key-flatpak#51

src/kolibri_gnome/kolibri_window.py

@@ -8,6 +8,7 @@
 from gi.repository import GObject
 from gi.repository import Gtk
 from gi.repository import WebKit
+from kolibri_app.config import BUILD_PROFILE
 
 from .kolibri_context import KolibriContext
 from .kolibri_webview import KolibriWebView
@@ -72,6 +73,9 @@ def __init__(
 
         self.set_default_size(DEFAULT_WIDTH, DEFAULT_HEIGHT)
 
+        if BUILD_PROFILE == "development":
+            self.add_css_class("devel")
+
         content_box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
         self.set_content(content_box)