Add custom css dark theme customization

[meel-hd]

Proposed changes

• The custom.css consumes the custom theme colors and use them.
• Colors are processed and passed to the template from CustomCssView.
• Fixed previous failing tests.

(depends on PR #12613)

Checklist

• [x] Lint and unit tests pass locally with my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added documentation to describe my feature.
• [x] I have squashed my commits into logic units.
• [x] I have described the changes in the commit messages.

Other information

@nijel splitting the PR to two was a good call. The issues introduced in the original PR are produced by custom.css. I am inspecting it. I will push later fixes.

Edit: Pushed fixes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.17%. Comparing base (64aa009) to head (4152a40). Report is 3679 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12794      +/-   ##
==========================================
- Coverage   91.17%   91.17%   -0.01%     
==========================================
  Files         596      596              
  Lines       61149    61164      +15     
  Branches     6345     6346       +1     
==========================================
+ Hits        55753    55766      +13     
- Misses       3745     3746       +1     
- Partials     1651     1652       +1     

Files with missing lines	Coverage Δ
weblate/configuration/tests.py	100.00% <100.00%> (ø)
weblate/configuration/views.py	90.69% <100.00%> (+2.12%)	:arrow_up:

... and 5 files with indirect coverage changes