[Docs] Remove theme overwrites from mermaid diagrams

[peter-csala]

Pull Request

The issue or feature being addressed

• [Feature Request] Dynamically change mermaid theme based on selected appearance mode dotnet/docfx#9306

Details on the issue fix or feature implementation

• The bug fix has been released today (link)
• We can remove the theme overwrites and enjoy the built-in support

Proof

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Light

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[martincostello]

Great work 😃

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (46978cf) 84.65% compared to head (5b4c30e) 84.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1705   +/-   ##
=======================================
  Coverage   84.65%   84.65%           
=======================================
  Files         306      306           
  Lines        6831     6831           
  Branches     1047     1047           
=======================================
  Hits         5783     5783           
  Misses        839      839           
  Partials      209      209           

Flag	Coverage Δ
linux	?
macos	84.65% <ø> (ø)
windows	84.65% <ø> (ø)

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