properly set background on file load

[rozyczko]

On loading a json project file, all parameters update correctly BUT background.
This fixes the issue

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.49%. Comparing base (5b0c694) to head (64e07cd).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/easyreflectometry/project.py	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #300      +/-   ##
===========================================
- Coverage    87.57%   87.49%   -0.09%     
===========================================
  Files           46       46              
  Lines         2784     2790       +6     
  Branches       338      339       +1     
===========================================
+ Hits          2438     2441       +3     
- Misses         261      263       +2     
- Partials        85       86       +1     

Flag	Coverage Δ
unittests	87.49% <62.50%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easyreflectometry/data/data_store.py	91.83% <100.00%> (-0.09%)	⬇️
src/easyreflectometry/project.py	76.36% <57.14%> (-0.28%)	⬇️

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