Fix Theme Properties referencing nonexistent file headers

[meszarosrob]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

It fixes the retrieval of theme information defined in the file header.

What is the current behavior? (You can also link to an open issue here)

We are trying to access the file header information using the "display name".

What is the new behavior (if this is a feature change)?

We are accessing the file header using the keys.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

Other information:

None.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c3886c3) 98.86% compared to head (c4c1130) 98.86%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #38   +/-   ##
=========================================
  Coverage     98.86%   98.86%           
  Complexity      189      189           
=========================================
  Files             9        9           
  Lines           528      528           
=========================================
  Hits            522      522           
  Misses            6        6           

Flag	Coverage Δ
unittests	98.86% <ø> (ø)

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

Files	Coverage Δ
src/Properties/ThemeProperties.php	90.62% <ø> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.