Fix/flashbar z index issue and pages background

[orangevolon]

Description

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.21%. Comparing base (cc75463) to head (3a649b9). Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2885   +/-   ##
=======================================
  Coverage   96.21%   96.21%           
=======================================
  Files         761      761           
  Lines       21502    21502           
  Branches     7354     7354           
=======================================
  Hits        20688    20688           
  Misses        806      806           
  Partials        8        8           

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