feat(ui): Instance customization for bootstrap 5 templates

[KarenKonou]

This PR updates the custom CSS template used by the appearance customization feature to work with the new Bootstrap 5 templates.

Part of #2916

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.51%. Comparing base (e6034da) to head (12c9c74).
⚠️ Report is 125 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/configuration/templatetags/colors.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16526   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files         666      667    +1     
  Lines       69581    69598   +17     
  Branches     7112     7112           
=======================================
+ Hits        63674    63690   +16     
- Misses       4135     4136    +1     
  Partials     1772     1772           

Files with missing lines	Coverage Δ
weblate/configuration/templatetags/colors.py	80.00% <80.00%> (ø)

... and 6 files with indirect coverage changes

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.