[Accessibility] Fix: screen reader does not announce theme change and nested nav label

[cheng-tan]

Why are these changes needed?

fix the accessibility issue that screen reader doesn't announce the theme when it changes

Related issue number

#5631 (13) (31) (59)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.64%. Comparing base (26364e3) to head (0208270).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6061   +/-   ##
=======================================
  Coverage   76.64%   76.64%           
=======================================
  Files         191      191           
  Lines       13201    13201           
=======================================
  Hits        10118    10118           
  Misses       3083     3083           

Flag	Coverage Δ
unittests	76.64% <ø> (ø)

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[peterychang]

lgtm