diff --git a/meld/meldwindow.py b/meld/meldwindow.py
index a5982ac7dee0bf71bfb7765fc9504125e348ec38..6b00ac355a5737e0e643137242b42813833a6f9a 100644
--- a/meld/meldwindow.py
+++ b/meld/meldwindow.py
@@ -23,7 +23,7 @@ from gi.repository import Gdk, Gio, GLib, Gtk
 # Import support module to get all builder-constructed widgets in the namespace
 import meld.ui.gladesupport  # noqa: F401
 import meld.ui.util
-from meld.conf import PROFILE, _
+from meld.conf import DATADIR_IS_UNINSTALLED, PROFILE, _
 from meld.const import (
     FILE_FILTER_ACTION_FORMAT,
     TEXT_FILTER_ACTION_FORMAT,
@@ -114,7 +114,7 @@ class MeldWindow(Gtk.ApplicationWindow):
         self.scheduler = LifoScheduler()
         self.scheduler.connect("runnable", self.on_scheduler_runnable)
 
-        if PROFILE != '':
+        if PROFILE != '' or DATADIR_IS_UNINSTALLED:
             style_context = self.get_style_context()
             style_context.add_class("devel")