Disable dark mode in Django admin UI.

1. pip install django-light
2. Add 'django_light', to INSTALLED_APPS before django.contrib.admin

This is a pure CSS tweak. The CSS file is injected via base.html in the extrastyle block. If you are using Jinja2 or override base.html in a way that doesn't call {{ block.super }} on that block, YMMV. But in the end all that is needed is for the browser to load <STATIC_URL>/django_light/django_light.css stylesheet.

Please report any problems via GitHub issues. PRs welcome.