css: Add dark theme

[nijel]

Proposed changes

• add CSS override for dark theme
• add javascript to toggle that
• allow to override this in the profile

Fixes #2969

Checklist

• 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.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Merging #9361 (84703c5) into main (b645f76) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9361   +/-   ##
=======================================
  Coverage   90.94%   90.94%           
=======================================
  Files         726      727    +1     
  Lines       54218    54223    +5     
  Branches     8352     8352           
=======================================
+ Hits        49307    49314    +7     
+ Misses       3405     3404    -1     
+ Partials     1506     1505    -1     

Impacted Files	Coverage Δ
weblate/accounts/forms.py	90.69% <ø> (ø)
weblate/accounts/tests/test_views.py	100.00% <ø> (ø)
weblate/accounts/migrations/0025_profile_theme.py	100.00% <100.00%> (ø)
weblate/accounts/models.py	88.61% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes