How can I override standard handler404, handler403, handler500 in Django?

I think you can not change the 404 page in DEBUG = True mode without difficulty.

There is a hint in the documentation (https://docs.djangoproject.com/en/dev/topics/http/views/#the-404-page-not-found-view):

If DEBUG is set to True (in your settings module), then your 404 view will never be used, and your URLconf will be displayed instead, with some debug information.

Try adding this to the bottom of your main urls.py:

if settings.DEBUG:
    urlpatterns += patterns('',
        (r'^404/$', TemplateResponse, {'template': '404.html'}))

Swap out the 404.html to the appropriate template you use, I believe 404.html is the default though. Then with debug=True you can test out your 404 page.

If you want to Test it out with Debug=True then you need this at the bottom of your main urls.py instead:

#Enable static for runserver with debug false
from django.conf import settings
if settings.DEBUG is False:   #if DEBUG is True it will be served automatically
    urlpatterns += patterns('',
            url(r'^static/(?P<path>.*)$', 'django.views.static.serve', {'document_root': settings.STATIC_ROOT}),
    )

When running with DEBUG = False, don't forget to collect static:

python manage.py collectstatic

Hope this helps, Cheers!