Django (grappelli): how add my own css to all the pages or how to extend admin's base.html?
If you want to change the appearance of the admin in general you should override admin templates. This is covered in details here: Overriding admin templates. Sometimes you can just extend the original admin file and then overwrite a block like {% block extrastyle %}{% endblock %} in django/contrib/admin/templates/admin/base.html as an example.
If your style is model specific you can add additional styles via the Media meta class in your admin.py. See an example here:

class MyModelAdmin(admin.ModelAdmin):
        class Media:
            js = ('js/admin/my_own_admin.js',)
            css = {
                 'all': ('css/admin/my_own_admin.css',)
            }
In Django grappelli, how can I add my own css files to all the admin pages? Or is there a way to extend admin's base.html template?