fix: add read_only_background property to schema for themes

[kushalshit27]

🔧 Changes

• Schema Validation Fix: Added read_only_background property to the theme colors schema.

📚 References

• Fixes Schema validation rejects read_only_background property that was exported by a0deploy itself #1179

🔬 Testing

Manual testing steps:

1. Export a tenant configuration: a0deploy export -c ./config.json -f yaml -o ./output
2. If the tenant has themes with read_only_background colors defined, verify the exported YAML includes them
3. Re-import the exported configuration without modifications: a0deploy import -c ./config.json -i ./output/tenant.yaml
4. Verify the import succeeds without schema validation errors

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.12%. Comparing base (4ce981f) to head (68b38c9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1184   +/-   ##
=======================================
  Coverage   80.12%   80.12%           
=======================================
  Files         134      134           
  Lines        5107     5107           
  Branches     1016     1016           
=======================================
  Hits         4092     4092           
  Misses        581      581           
  Partials      434      434           

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