Fix header flash

[fiskus]

1. Actual fix - apply M.ThemeProvider on upper level, so M.makeStyles uses correct theme
2. Also corrected M.AppBar background, but it is covered by custom styles anyway

• Changelog entry (skip if change is not significant to end users, e.g. docs only)

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• To see the specific tasks where the Asana app for GitHub is being used, see below:
  • https://app.asana.com/0/0/1204106012781758

[codecov]

Codecov Report

Merging #3487 (887b278) into master (5ce4d69) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3487      +/-   ##
==========================================
- Coverage   36.15%   36.15%   -0.01%     
==========================================
  Files         684      684              
  Lines       29929    29931       +2     
  Branches     4414     4414              
==========================================
  Hits        10822    10822              
- Misses      17939    17941       +2     
  Partials     1168     1168              

Flag	Coverage Δ
api-python	91.35% <ø> (ø)
catalog	10.19% <0.00%> (-0.01%)	⬇️
lambda	86.03% <ø> (ø)

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

Impacted Files	Coverage Δ
catalog/app/constants/style.js	0.00% <ø> (ø)
catalog/app/containers/NavBar/NavBar.tsx	0.00% <0.00%> (ø)

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