change appbar style

[sensuikan1973]

On flutter X.Y, the appbar default style has been changed.
So, I specify the style expressly.

[codecov-commenter]

Codecov Report

Merging #1644 (cfbcb0f) into main (6f49043) will increase coverage by 0.46%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1644      +/-   ##
==========================================
+ Coverage   93.54%   94.01%   +0.46%     
==========================================
  Files          52       52              
  Lines        1503     1503              
==========================================
+ Hits         1406     1413       +7     
+ Misses         97       90       -7     

Flag	Coverage Δ
integration_test	70.92% <ø> (+0.46%)	⬆️
macos	93.08% <ø> (?)
ubuntu	93.47% <ø> (ø)
widget_test	90.41% <ø> (+0.47%)	⬆️
windows	93.47% <ø> (ø)

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Files	Coverage Δ
lib/home/home.dart	96.96% <ø> (ø)

... and 2 files with indirect coverage changes