enhance: move background color from body to html to enable developer override

[zombieJ]

resolve #6363

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (482d15a) 91.56% compared to head (f016f83) 91.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6364   +/-   ##
=======================================
  Coverage   91.56%   91.56%           
=======================================
  Files         306      306           
  Lines        6590     6590           
  Branches     1626     1626           
=======================================
  Hits         6034     6034           
  Misses        521      521           
  Partials       35       35           

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[github-actions]

Size Change: +19 B (0%)

Total Size: 355 kB

Filename	Size	Change
./lib/bundle/antd-mobile.compatible.umd.js	189 kB	+12 B (0%)
./lib/bundle/style.css	14.8 kB	+7 B (0%)

ℹ️ View Unchanged

Filename	Size
./lib/bundle/antd-mobile.umd.js	150 kB
./lib/bundle/css-vars-patch.css	1.42 kB

_{compressed-size-action}

[github-actions]

PR preview has been successfully built and deployed to https://antd-mobile-preview-pr-6364.surge.sh