Dark mode support for django CMS 3.11/django admin (since version 3.1)

[fsbraun]

Description

This PR implements css changes to reflect django CMS dark mode (if djangocms-admin-style is used) or Django dark mode (if djangocms-admin-style is not used) in a coherent way.

SASS constants are replaced by css variables of django CMS or of Django 3.1+ as a fallback.

No changes to python or js code.

I would appreciate if the PR is user tested for color experience. Install by

pip install git+https://github.com/fsbraun/django-filer.git@feat/darkmode-support#egg=django-filer

Related resources

• django CMS version 3.11

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic (N/A)
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

Merging #1315 (e1436d7) into master (dc6afa0) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1315   +/-   ##
=======================================
  Coverage   72.21%   72.21%           
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  Files          72       72           
  Lines        3239     3239           
  Branches      563      563           
=======================================
  Hits         2339     2339           
  Misses        734      734           
  Partials      166      166           

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